well~

you can add friends (mine is "exotichicken" so you can add me ^^)
lj isn't so much abouy socializing—it's more for reading and writing so don't expect an aff environment

there are communties (like groups) where people post up stories/oneshots
usually each community focuses on one band/pairing/genre
it depends on what you like
just google search them and you can track these communities to get their updates
you can even make your own community!

now, your profile and your journal are separate
you can really only change your journal theme
look up "livejournal" themes and you'll get some pretty stuff

on you profile you can just add some basic info
personally, your journal should be more important

usually when i writer posts something up, they do an "lj cut"
basically, there's a setting in the toolbar (as you write) where you make a "cut" and anything that goes between the scissor-looking majinhers will be hidden on your journal until someone clicks on the link that opens the full entry

this is a basic overview

sorry if it's confusing
i'm still getting the hang of lj myself