So im planning on having sex soon

So I'm 17 years old and I'm planning on having sex with my boyfriend once ... So I'm not getting on any birth control but I will be using a sponge a condom spermicide and a plan b just in case I'm scared. I'm very nervous cause I do not wanna be pregnant I'm gonna be having sex 6 days after my fertile window. Any advice?? Do you think I will get pregnant?