as stated above, you are not eligible to take the boards (board eligible/be) until you complete residency. if you take and pass the boards, you are then board certified (bc).
however, you do not need to be board eligible nor board certified to practice medicine. in other words, in plenty of states, you can start practicing after you complete your internship year and pass all 3 steps. it may be difficult to get hospital privileges, and you likely won't be able to secure contracts on your own, but you can certainly work in urgent care, or be a staff physician in someone's office.