You get points when you post in the forum. You can use them to play in the contests - click on the FREE CONTESTS banner in the right column (below the leaderboards). But it's too late for this month's POD (pick of the day) contests so you might want to hold until next.
Just like endy said you get points for posting and can use them to enter the contests. He's also right in that the POD contests require 20 picks minimum so it's too late to start those this month but the daily streaker contest is always open....good luck!