Take your time, relax, be clean down there and enjoy.
For your first time you don't want to be on your back. That tends to compact your anal region. You're going to want to be as roomy as possible.
Lay on your stomach with a pillow under your groin. This will help stretch you out and goive more room for the cock to go in and out.
Make sure you are lubed AS MUCH AS POSSIBLE.
When you feel the head of the cock pushing against your ass, push down like you are going to the bathroom. Let it slide in as far as it is comfortable and then rest and let your sphincter muscle get used to the stretching. REMEMBER TO BREATH! Most newbies forget to do that.
Push down again and let more cock into you. Repeat until it's all the way in. Some people feel pain but it does go away as you get used to it. Some feel pleasure right away.
When the actual fucking starts, rythem is important. As he pushes in, you push down. As he pulls out you relax and let the sphincter close so it's nice and tight around his cock.
Once you feel comfortable enough taking a cock in that position, try having him on his back and you ride his cock. This will give you more control over how fast he can go in and out of your ass.
Hope this helps.
Sklar
P.S. I'm up in Everett.