If you place in the top 70 in a weekly tournament, you will receive credits. It won't be much unless you place pretty high up, the most I've gotten so far has been 10.
A much easier way to get credits is to do surveys. You can get to them in the "GET CREDITS" tab up at the top, under "COMPLETE OFFERS". The Ampario surveys are generally considered to be the best ones to do. You can only attempt one each of the short and the long Ampario surveys each day, and qualifying for them can be tricky. I've gotten somewhere around 1500 credits by doing surveys.
Once you have credits, you may try your hand at either match play competition, where you wager credits on the match; or the Ready Go tournaments. I would suggest not trying those until you are pretty confident in your abilities.