Creating A Journal
Creating a journal allows you to leave comments on the entries from some journals that do not allow anonymous commenting, join communities, add users as "friends", be added as a "friend" by other users, read entries of others on your "friends page", read certain privacy-restricted entries from other users that allow you to do so, as well as post entries of your own.
If you haven't already created a new journal, you are likely to want to create one after hanging around the site for a while. You can certainly use lots of parts of LiveJournal without having an account, but it's helpful to have one even if you aren't planning on sharing the personal details of your life with the world. This FAQ covers the account creation process in brief. You start out from the Create New Journal page and follow the step-by-step instructions. You will need to have a 15-character or shorter username in mind. This is the username that your fellow users will see. You will also need a valid email address. When possible, use an email address that you will have control of forever, since the email address used to set up the account is important, and the first email address used cannot be removed from the account
Using Your Own Journal
There are limitless ways to use your account. There is no one correct way to do it. Some people post public accounts of their own lives, or their thoughts on a variety of subjects, or a narrowly focused category of subjects. Some people are only signed up to read content that others publish, either through their friends page or by visiting the journal directly. Some people use LiveJournal as a content management system to publish a creative project. Some people use it to make new friends; some use it solely to communicate with people they know face-to-face. Some people are just here to keep in touch with one person; some people manage an intricate social network. Some people disable comments on their journal entries entirely or restrict them to only users listed as their friends; some allow the general public to have broad-ranging discussions that are only tangentially related to the topic of their entry. Almost anything goes. (Journals created to spam, among some other things, do not go.)
Once you have created your account, you can post entries to it, customize the main area of your journal, edit your profile, and upload userpics. None of these are required, but give other users a little more information about you and your presence on LiveJournal. When posting any information to LiveJournal, please remember that any information posted publicly may be seen by any other LiveJournal user, and potentially indexed by a search engine. Use appropriate discretion when revealing any personal information about yourself or others in a public location, and when sharing information with people you have met.
You may choose to subscribe to (or unsubscribe from) notifications for a number of events. Most notifications are sent to your inbox, and can be managed from your Subscriptions page.
Privacy and Security
When created, journals by default do not have the most-restrictive settings for contact and visibility set. If you plan to interact with other users, you may wish to leave the settings at their defaults until you decide what you will be doing with your account. If you already know that you would like to have a more private experience, these are some of the settings you should know about. You may set your journal to automatically set the security of all future entries to Friends-Only or Private. You may restrict who may comment to your journal. You may choose to log IP addresses of commenters. These settings and more are available in the Privacy section of the My Account Settings page (currently beta only).
Since it is possible to remain logged in on multiple computers, you may review and cancel any currently logged in session from the Manage Login Sessions page (available from the History section of the My Account Settings page). If you have logged in and not logged out from multiple locations or multiple browsers, it can be perfectly normal to see multiple logged in sessions. However, if you suspect someone has maliciously gained access to your account, please immediately expire all other sessions, make sure you control all email addresses on the account, change your password, ensure your secret question is secret, and contact the Abuse Team.
Interacting With Others
While you may choose to use LiveJournal in complete isolation from other users if you wish, LiveJournal can be a highly social service. People you know may have already given you their LiveJournal username, you may locate people you already know on LiveJournal, and you can meet new people through LiveJournal -- through existing friends, through communities, through shared interests, and at random. Other users may also contact you, if you wish to allow this.
You may view any user's journal at http://exampleusername.livejournal.com, and their profile at http://exampleusername.livejournal.com/profile, replacing "exampleusername" with their username. Many users and communities list important information about themselves in the Bio section of their profile, and/or in a post dated into the future in their journal.
The primary way of interacting with other users is to add them to your friends list, so you may view their entries on your friends page. The friends page shows the recent posts from the other journals you are watching all in one convenient place, so you can read them there.
Many communities offer the ability to join them, although some do not. It is usually necessary to join a community to make top-level posts to it, although it is not usually necessary to join it to make comments to top-level posts from other users. (However, these settings may be changed by the users maintaining the community.) You may also watch a community by adding it to your friends list.
Adding a user as a friend will also allow you to make posts that are invisible to users that you have not listed as a friend. This is a useful feature if you would like to talk about topics that you are not comfortable discussing in public. As always, use discretion about the information you share even in protected posts, as it can be difficult to remove personal information from the internet if a friend or former friend is careless with it and somehow makes it public.
Although adding another user to read their entries on your friends page, and allowing them to read your protected entries, is referred to as adding them as a "friend", this is not actually intended to imply that you consider them friends in your offline life. Similarly, if someone you do not know adds you as a friend, this is not intended to imply that they believe that you are actual friends, just that they would like to read your entries and/or allow you to read their protected entries.
If another user makes comments to your journal that you feel are inappropriate, you may ban them from commenting to your journal. However, this will not prevent them from reading your public entries, or replying to an entry or comment you have made in a community or other user's journal.
As a community, LiveJournal has evolved some interesting social customs, some of which I have collected into an etiquette guide, and some of which I've violated myself, or watched being violated with poor results. I have also written an essay covering some of the multi-faceted customs involving adding others as friends. These are my observations only, and may be different among your social circle. Other of my LiveJournal-related essays, announcements and musings can be found in my entries tagged "LJ".
Updates, Known Issues, Technical Problems, and Feedback
LiveJournal has a number of areas that are updated with information about the site, depending on topic. A listing of the major communities can be found at the top of the News page. The news journal updates on approximately a monthly basis, and attempts to cover major changes and items of interest. lj_maintenance announces planned downtimes, and other performance-related information. lj_releases summarizes new changes to LiveJournal's code and features.
Known technical problems with the site (and resolutions and/or estimated time of resolution) are listed in the Big Blue Box in the Support Area. This covers issues that are not severe or widespread enough to be listed on the off-site Status page (also available at status for your friends page).
Most questions about the operation of the site are also addressed through the Support Area. Bugs and technical problems are also reported through opening a support request here.
The Contact Us page also has an option to contact Support, as well as Abuse, Billing, Press, Partnerships, Suggestions, and Feedback. The Suggestions option is used to suggest new technical features or suggest changes or improvements to existing technical features. Suggestions are reviewed by employees or senior Support volunteers, and (if approved) results in a post to the suggestions community. The Feedback option is used for opinions on site design, policy, and administrative decisions, or suggestions for changes to any of these. Staff requests for feedback are posted to lj_feedback.
...And Much, Much More!
There is much more to LiveJournal than I've covered in this brief introduction, but this should give you a grounding in where to find information on the basics. Please enjoy your stay on LiveJournal, and I hope you have as much fun here as I have over the years.