First of all, let me establish the most important aspect of giving a blow job -- confidence! A lot of women are self-conscious when it comes to giving head. While I can certainly understand being unsure of yourself, that's something you have to get over quickly in order to improve. Even if you're not the world's greatest, if he's getting a blowjob, he's pretty much already on Cloud 9. I'm not saying there's no wrong way to give head, because there is. I'm just saying that if his dick is in your mouth, for him, things are already off to a great start.
If you still feel unsure, tell your partner that you're nervous. Most guys will be not only understanding but turned on by what they'll see as your "innocence." They will be more than happy to guide you to what they like.
Also important is something that many people overlook -- position. Both partners have to be comfortable, which admittedly can be difficult at times. Also, the right position can give your man the perfect view of your handiwork. Here are a few positions that work for us:
1. He kneels on the bed or sits cross-legged, leaning back so I have plenty of access. I lay propped up on my side on the bed. This puts me in the perfect position in case he wants to finger me.
2. He sits on a low couch or chair and I sit on the floor, leaning into his legs to support myself.
3. He stands up and I kneel on a pillow.
4. He lays down (with his head propped with a pillow) and I lay between his legs with my arms propped up.
A few more somewhat silly things before you begin: Taking frequent breaks is not only a necessity for you (unless you enjoy lockjaw and chapped lips), it can also allow him to last longer and build up a more explosive orgasm. Don't feel bad if you need to stop -- just don't do it abruptly. Second, you may want to have what to me are a few sex "essentials" on hand: water/drink, lip balm, lube, and of course, a hair band to hold hair away from your face.
I like to start off by gently holding his cock in my hand (hard or not) and slowing kissing and licking just the head. The point is to establish to him that you love his cock would love nothing more than to give him a blowjob. At this point, you can also stroke him very, very gently. The lightest touch can feel really good as he's anticipating things getting more intense.