Okay... first things first... have you ever put anything in your ass? Go do that. Some people like fingers. If you don't, don't go sticking your fingers in yourself hoping they'll feel nice. Just get yourself a dildo instead. A normal size dildo, not something ridiculous
This putting-things-in-your-ass thing is important because it helps you realize that, while the ass is very sensitive in certain ways and kinda fascinating in certain ways, it's not magic, it's not going to bloom into a beautiful rose when the right man touches it (well, not immediately) and when you do something a little painful to it (such as use your fingers or toy too hard or fast) it ain't gonna kill ya.
Also you get to practice Not Touching Things with your dirty hand and/or toy after it's been in a butt, until you've washed it (or wrapped it up in a *ahem* cumrag to deal with later). How fun! Seriously though, once you get used to this part of the etiquette, everything becomes more comfortable.
Okay... ssssex.
You may not come or even feel very close to coming, at first. Some guys hardly get there at all. So don't worry about it! Especially early on, you may be thinking about other things entirely, like the cock that's fucking your ass and what that all feels like.
What will it feel like? Well, he'll have his way with you

Until you're aware of what makes you come and can guide him into screwing you that way, or if you're someone who doesn't go for orgasm as a bottom, there's nothing wrong with just enjoying his lust and taking it like a good bitch, *cough* I mean bottom. Allowing a man to finish inside you takes a certain amount of submission I think, in the moment, but it is a nice feeling as well to be able to pull an orgasm out of a man, if you really want it, it can have an effect on him.
Here's the deal, you have a series of rings inside your colon. The first one is the one nearest the outside, your sphincter. Then after you pass by the prostate area which feels really nice when it's with someone you trust, there's another inner ring, and after shortly that one there's another, and another, and so on. These are associated with ever deeper levels of emotional vulnerability. It's perfectly normal (if it is someone with whom you are still building trust, or if it's a bad day for you, or if you just don't feel 100%) that you can let him through some rings but not all. Don't worry about being closed off to him, it's not fun to try forcing it (emotionally or physically). If it's going to happen it will at some point.
Things to experiment with if you're lost include:
* Embracing him in different ways
* Curling your spine
* Making yourself looser or tighter
* Trying different metaphors for motion between two bodies: riding him vs. fighting him vs. dancing with him vs. begging for him vs. claiming him vs. ... ...
You can always breathe faster, hyperventilation helps with pain tolerance, but it's not necessary a good idea to breathe faster on purpose just to make yourself lightheaded because it can bring you out of sync with your guy.
It is a good idea to start with just a penetration, just sitting on him or having him enter you, and going farther and farther just reacting to the coitus itself, not trying to move back n forth or any kind of thrusting. Try joining as far as is comfortable or for as long as you can stand to do this simple motion rather than something more complex

Just breathe and feel really aroused and reallly turned on and enjoy the feelings! I find that this helps because the bottom has no set 'depth' that he can always take, it really depends on his body and his mood and the time of day and everything and on and on, and it helps both of you become more comfortable when you find out first what he is happy to hold on to right now, and THEN start to see how much he can take in stronger motions.
Unless he's a masochist or really submissive, don't actually hurt or try to injure the bottom, he won't be happy.
You can never make your dick flare Too full of blood.

If you bottom out and the bottom still wants more, then fold him up like a pretzel so you can go deeper, or start really vibrating his butt with the force of your hip slams on his tailbone, it's amazing what some thumps up the spine can do to a boy.
EDIT: LLLLLLLLUUUUUUUUUBBBBEEE.