For the beginning make sure you've filled your payment details. After that go to the section Offers in your account and choose an offer you are interested in.
Read the rules&description carefully and if everything's OK for you, send a request for activation with mentioning of your traffic source.
Keep in view that not all the offers can be available right away. If you are into ppl-offers, then after sending a request you'll get a ticket. You have to read it very carefully and follow all the mentioned information in it.
After performing these simple actions you'll be informed in your account about approval or rejection of your request.