When you are new to Stackexchange, it can be hard to know what all you can and should do at first. Here are some ideas:
Posting. Everyone who comitted to this site is encouraged to make 10 posts (questions or answers). This will formally fulfill your initial commitment, but you can continue to use the site normally afterwards.
Voting. Everyone is allowed 40 votes a day. If you upvote every post that seems good and downvote everything you think doesn't belong, you can use up most of these votes easily provide helpful feedback for others.
Flagging and closing (per Thomas' suggestion). Flags are used to alert moderators to offensive or spam remsrks and posts and to answers that should be comments, among other things. Close votes are for questions that need to be reworded to fit the site or which just don't fit at all.
Badges. Most badges are there to encourage you to try out every useful feature. Chat, meta, and votes are all described on the Badges page.
Sharing. Once the site is out of private beta, you can share links to great posts. Highlighting our best content is the most effective way to encourage others to participate.