You should be able to join contests immediately after you sign up. 4 Contests are automatically assigned to you immediately upon sign up. Additional contests will be assigned to you on a daily basis.
You can check all contests assigned to you here.
Once you participate in few contests, and receive good ratings on your submissions, your account will be automatically upgraded to full access. Once you have full access, you can join all open contests (with the exception of contests that are only open to Tier A creatives).
Check out this video if you're having trouble finding your assigned contests: