Don't be nervous or scared! You need yourself to be as relaxed as you can possibly be or else you will be quite uncomfortable during and after the sex.
Make sure the guy topping understands he has to go very slow, use a condum, and lots of lube.
It is best if you work your way up to 3 fingers in your ass before he tries his dick otherwise it will hurt like hell.
Also when he puts his dick in, push down with your ass muscles like your going to the bathroom. It loosens everything up and makes the pain go away much quicker. When you guys really get into it and your comfortable with and enjoying the fucking, clamp down with your ass muscles every now and then... it drives the tops crazy.
Bottoming is awesome when done right, and is my favorite thing to do; in fact bottoming is better than topping in my opinion. The bottom has so much control over the experience for both guys. A good bottom is definitely a good find.
Good luck dude!