It takes getting used to. It looks a bit chaotic at first. But you can click your profile, and add stuff there. Games you own, have for sale/trade etc.
You can search for games, but also for users etc. from the search bar with the rolldown menu. Then you can add someone to your friends with the small down arrow above the persons name.
When you check a game's info page, way down, you see +add a copy to your collection. Click this if you own it / want it / want to play it etc. You can check/uncheck the checkboxes and fill up your list that way.
There's also a gallery of images you can upload to your profile.
If you get enough GeekGold, you can buy an avatar. You can get GG by simply donating to the website ($1=1GG), but that's expensive. Avatar = 30GG!
But veteran BGG people have more GG than they know what to do with, so they can tip you for making good posts, or useful posts/reviews etc. There's even a topic where people saving up for an avatar can ask for GG. The community is friendly enough to provide you with some GG just for that purpose. So post something funny/nice/helpful and they will tip you.
It's fun to get positive feedback from blog entries or reviews. Just take your time to write something worth reading, and get to know the site and community a bit before you post.
There are also regular contests where you might win a copy of a game. When I'm not reading Ye Olde Inn, I'm lurking over there nowadays.