Get lube and buy a douche. Clean your ass out with the douche by putting in warm water until it's full then put it in your ass and squeeze the water in there. Lube the nuzzle you're inserting to make it easier to put in. Move around, jump, shake a bit then go to sit on the toilet. Squeeze the water out of your ass as if you were taking a dump and look to see what comes out. If you see crap or any time of color, repeat the process. Flush the toilet and keep on repeating the cleaning until the water comes out clear when you release water. This whole process may take a very long time, sometimes up to an hour, sometimes less, sometimes more. Depends on different things but it's worth it.
After you're cleaned out, lube your fingers and try to open yourself up with your fingers. Finger yourself and try to ease yourself into it. The best thing to do is to relax. Try not to tense up. Tensing up will make receiving anything extremely difficult. Go at your own pace but don't chicken out or give up. Keep fingering, add another finger and breathe and relax. It should start feeling good. The pain may be something you'll get used to but it'll get pleasurable. Do it until you feel comfortable.
When with your boyfriend, the overall best thing is to relax, breathe, put on lots of lube, and have him be as comforting. He should know it's your first time and it may not be easy for you. Allow him to push your limits, but if it's really painful or something you don't feel is right, don't do it.
And I hope you get to experience topping him. It's awesome.
