Sex while in fertile window?

So, my boyfriend and I want to have sex but the day we're going to have sex is my "Fertile window" I'm not on birth control yet, I haven't made an appointment with my DR, but if he wears a condom and I'm not on birth control yet what are my chances of getting pregnant? 
Also if I were to begin birth control let's say 4-5 days before we have sex would the birth control be effective in that short amount of time?