First Time Tips???

Heyyy!! Me and my girlfriend have been together for a while, and have finally decided it’s the time to move up in our relationship. I’m a girl and so is she (I’m bisexual, she’s a lesbian), but the problem is that she’s had sex before but I haven’t. I’m really nervous, and have no idea what to do!! She wants me to top, and I have soooo many worries that I’m going to screw things up. We’re both pretty young, but we’ve been together for a while so yeah. Any advice?