as mentioned by the previous user, this depends on what sort of BC u on. but usually Drs ask u to use condoms for extra protection during the first 7 days to avoid any surprises.
it depends on the type of birth control is it pills, shot, patch, iud or something else? and when you start taking them in your cycle some protect right away or if taken within 5 days of the start your period some you need to use back up with planned parenthood has a great list of all types of birth control and when you need to start taking them and how long it takes for them to become effective