How long after taking the birth control pill does it become effective. I am taking the brand alesse if that matters.

What did your gyn sa. Say that is one of the main things he or she should have told u, you should use backup for the first pack, by the time u get to the placebo pills at the end u should be protected but if u are 18 and not married you should be using condoms also to protect against stds that i see everyday in my offcie.
Generally immediate. If the pill is started in the first 5-7 days from the onset of a menstrual period, it usually stops the next ovulation. To be on the safe side, use a backup method for 2 weeks. Remember, the pill does not prevent any diseases, so using condoms is generally recommended until marriage which will also serve as a back up method.