Stunned by something a girl friend told me... how should I handle this?

I met a girl about 6 months ago. We connected when we first met, and quickly became each other's best friend. We've told each other a lot about our lives, the good, bad, ugly and all that jazz. We trust each other as much as two people possibly can.

We've always been flirty around each other, but we've respected each other's space and have never really seriously talked about trying to be together or exploring each other physically. We have both expressed interest in being in a relationship, which is great.

Last night, we were texting each other and she mentioned she'd like to have sex with me (in a serious tone). I was taken back even though I had a good idea she wanted to try that out.

I've been hesitant about that kind of thing before. I did not grow up in a great environment in the emotional sense, having been bullied quite a bit during my middle school years. That bullying, and a few other personal things, has caused me to not trust people - or myself - as much as I should. I have virtually no self-confidence whatsoever.

I've never been in a relationship, and only recently began discovering my personal needs. I'm not the kind of guy who puts himself first, although I'm happy that I'm starting to change that. We know each other's wants & needs, but I still have a lot of self-doubt.

How do I keep calm and collected about this & not let it get to me? Thanks for the help!