Once you've set your preferences up, operating base and so on, search for a team and check the list for one that's Level 50 or above. If there aren't any, you can set up your personal data with a different operating base and/or play type and try again.
If their Enlistment Method is set to free, you should get in without waiting for approval, and get the achievement pop soon after, along with Rookie and Team Level 10.
Then it's up to you if you want to stick with them or shop around for another team under the Manage Enlist. screen under the Team Menu.