Fortunately, it is somewhat easier to give a guy oral sex. There is not too much you need to know. Basically, put penis into mouth, move up and down (in and out) until desired effect is achieved.
Even though it is basically easy to do, there are some things you should know/consider beforehand.
First of all, be careful of your teeth!! I have heard many tales of pain and woe because of a girl (or guy) who was not careful with the placement of their teeth. Running your teeth - VERY GENTLY - over the outside of a penis can and usually will feel good for a guy, but do it too hard, or more than one time, and you can cause a substantial amount of pain.
Second, what do you do with your tongue? The tongue is an irreplaceable instrument when giving a guy head. Use it to stroke and massage the penis and it slips in and out, and tickle the head of his penis with it. Lick his balls. Be creative. Guys are, on the most part, pretty perverted (no offense guys, but you are). Don't be held back by worrying a guy might think you're weird because you get a little creative.
Third, what if he's really big and you can't get him into your mouth all the way?? Well, this is NO big deal. REALLY. Basically, most of the nerves in a guy's penis is located in the first half of his penis, and mostly they are on the head of the penis (that helmet looking thing). So if it doesn't go into your mouth all the way it's no big whoop. However, I have myself discovered a little trick to deal with my fiance's well-sized organ. What I do is slobber a lot at first and get his penis all wet and slippery, or even lick my hand, and then use that on the base part of his penis, sliding it up and down to match the rhythm and direction that his penis is moving in and out of my mouth. Because most guys are big porno watchers - to at least some degree - so having painted red nails is a good effect when doing this.
Fourth, the issue of ejaculation. What to do? Basically, there are three options. You can swallow, spit, or duck and let it fly. With swallowing, there isn't much in the way of health concern. The sperm won't make you sick to your stomach or anything, it is basically protein. And usually it doesn't taste bad either. I'm not saying it tastes GOOD, but it usually doesn't taste bad. With some experience you'll learn that different guys will taste different, and that one guy can taste different at different times, depending on what he's eaten, among other things. However, though it is rare, you can get a sexually transmitted disease from letting him cum into your mouth. That is, IF you have an open cut in your mouth, and IF the sperm comes into contact with the open cut, AND can penetrate the cut. Basically, even if he is infected, chances are slim. BUT, BETTER SAFE THAN SORRY. You can use a non-lubricated condom if you are concerned of the health risks. Next option, you can spit. Basically he's still cumming in your mouth and you'll still taste it, but you spit it out into a tissue or washcloth or something. I think personally that this option is pretty lame. If you're going to let him cum into your mouth and taste it anyway, you might as well swallow it down. You will actually taste it for less time if you swallow than if you let it sit in there until you grab whatever it is you're spitting into (and he'll be thrilled, to boot). Third, you can duck and let it fly. This option is actually a lot of fun, but it is the messiest of the three. He may just gloop a little onto his stomach, or he could cum onto you - breasts are a popular choice, there's that porn film influence again. Or if he really fires he may hit you, or himself, in the eye, or get the wall or ceiling or any other nearby object. It is definitely interesting to see where it will end up. However, whatever you choose, you should decide in advance so you can be prepared when the moment arrives.
Thanks! Come again!