How do I know that I'm ready for sex?

I've always wanted to wait until marriage before having sex, but now I'm beginning to feel differently. I'm 18 and I've been dating my boyfriend for almost 9 months now. He isn't a virgin but has not pressured me in any way. I'm so head over heels in love with him and I just don't know if I can wait any longer. But how do I know for sure that I'm ready?

side note: just in case we do end up having sex, I'm getting an IUD next week and he's getting an STD screen.