LJ users each have their own journal and user name. Each journal has the same general features:
'Recent Entries' clicking this will take you to the actual journal itself. The user can determine how many entries appear on each page. The entries go in order from most recent to distant past.
'Archive' is a calendar page showing how many posts were made on any given day, and provide a link to the journal entries for that day.
'Friends' takes you to a page which shows the recent posts of the user's LJ 'friends.' Friends can be people you know, people whose journals you stumbled on and find interesting, communities you want to watch, and so on. By 'friending' someone, you then have the convenience of clicking your 'friends' button and reading only the journals you want to read - without having to bookmark each individual's journal URL. Try it when you're done reading this! Click 'friends' and see what my user friends have posted. Then click their user name and go to their journal. Fun, huh? ;-)
'User Info' is a profile page of information each user enters for themself. Some profiles are very detailed and contain bios and pictures and links and rules and mantras...and others are bare bones and tell you nothing about the user at all.
On the 'user info' page, you can enter your interests and each interest which is shared by another user is highlighted as a link. If you follow, say, the link on my profile page to 'Maroon 5,' you will find there are hundreds of communities and users who list Maroon 5 as an interest also. So, it is pretty easy to find other users with similar interests as you. It is also a very fun challenge to try to come up with an interest that no one else has on LJ ;-)
To post in a community, you first need a journal and user name, then you need to join the community by going to its 'user info' page and clicking the link to join. Once you are in the community, your 'friends' page will have all the community posts in it - which means you don't have to visit the community at all! For chronic bloggers, this is a great feature and makes joining many different communities very easy to keep track of all on your 'friends' page.
Ok, enough tips for today! Have fun!