Aw hun, it's alright.
A first kiss is a big deal, but if you don't feel that you're ready, reschedule. You should be excited for this, but it's normal for you to feel nervous. If the guy you like knows it's your first kiss (especially if it's his too) he should know that you're nervous. Once you kiss him, it'll relieve a lot of awkwardness, and the next time you kiss someone it'll be a lot more romantic and enjoyable.
I'm sure it will be fine! Good luck.