Right, to start with please pardon my non-erotic and mechanical descriptions.:smile:
Make sure your boyfriend feels totally comfortable before you have anal sex. If he feels bloated or otherwise full have other forms of sex and wait until the next time he is ready. Avoid enemas as it might not be completely expelled and might leak before or during sex.
Spread some lube on your finger and play with his anus. Don't insert more than one finger even if your cock could be 3 fingers' width. Cocks and fingers aren't the same thing. If you don't like fingerplay buttplugs are also good just make sure you don't share it.
Use a condom. Personally I do not like to lubricate the inside of the condom or my cock before putting it on.
After inserting the head of your cock you will encounter resistance. Slowly and gently push against this at regular intervals until it gives naturally. This might take over a minute. Unlike what porn might make you believe, it is painful especially the first few times until he gets used to the sensation. If you take things slow and let things stretch gradually then he might feel enough pleasure to forget the discomfort. The prostate is kind of variable, some men say they can feel pleasure from it, others don't.
Try not to remove your cock too often as air might get in and interfere with your boyfriend's pleasure (this includes avoiding the temptation to try all the positions you know in one session!). If you do, apply more lube or put on a new condom if it is soiled or weakened.
If you want to avoid deep penetration spooning is the best followed by the scissors position ie him on his side and you straddling his leg whilst supporting the other on your shoulder. Vary your thrusting depth and speed to see what you both like best but usually this becomes subconsicous as it starts to feel better and better. If you want to let him control the depth then get him to sit on you with his knees pointing forward with feet facing away from your bodies. You can help support him by putting your hands under his thighs. Alternately he can initially squat with his back facing you then support his weight on all fours after penetration.
For deeper penetration doggy or you on top is good. You on top provides more intimacy but with the doggy you are more likely to slap your balls on his buttcheeks which is heavenly. A good variation of both these positions is with your boyfriend at the edge of the bed and you standing on your feet. You can also try them on the floor to see if you prefer it less bouncy! If you are finding it difficult to cum then the ultimate is him lying on his abs with legs together and you on top and he should grind his ass up in synch with your out stroke so you get constant stimulation. The extra friction from his buttcheeks and his grinding usually speeds things up! Also depending on the mood you're both in you might want lots of close contact and kisses throughout your session or focus more on genital contact if you're feeling wild and pornstarish.
In some cases you might be thrusting your cock numb and raw and still not be able to cum. If your boyfriend is really sore and bored by then just pull out and start again later, finish by hand or just rest. Sex feels unfulfilling without cumming for many men but sometimes you just can't force it so just chill and don't feel bad about yourself. On the other hand if you cum too soon it means you will be ready for round two which will probably be better than the first.
Finally to be a good top you should also bottom at least once to know what feels good and what doesn't or, worse, really hurts. If your boyfriend is traumatised by his first experience he may well never want to bottom again.
Hope this is useful!