Since you're in the Bi forums that means you're a boy?

Before you start:
-Find some lube, like hand lotion or KY jelly.
-Explore yourself by yourself before you do anything with your boyfriend.
-Shower and get that area really clean. Push some soap up in there to get it cleaner.
When you're ready:
-Give him the lube and make sure he goes really slow and starts small.
-Once you're comfortable with one finger, you can work your way up to two and three.
-No matter how slow you go, there are likely to be some painful moments. Just try to relax through them and open up as much as you can.

If you want more tips PM me, I can chat too.