Very few men will complain about getting a blow job, no matter how unskilled you may think you are. As long as sharp fingernails, rough skin and teeth are all kept well away from the ‘meat of the matter’, you're likely to get a welcome reception. However, there are some ways to enhance your technique – and keep things fresh.
First off, remember a good BJ is as much about the hands as the mouth. Keeping everything well lubricated will help too. Keep a pint of water next to the bed to rehydrate as you go and invest in flavoured lube. Not only will it help your hands glide over him more smoothly, but it will also make oral easier.
Oral transmission of STIs including gonorrhea and syphilis is on the increase, so use a condom for oral unless you've both been tested.
Vary the position to help you reach the bits that make him go 'oh!
Speed, depth and motion all make a difference to oral. Start slowly, and gradually get faster, seeing which speed elicits the most favourable response. Porn tends to depict hard, fast blow jobs but a more sensual approach can go down very nicely. Some men enjoy it if you speed your movements as climax nears. Follow the motions of his hips to help you set the appropriate pace or just ask him what he likes.
You may want to ask your partner not to use his hands on your head to guide you (unless you like it, of course). If you set the pace, it will be easier to ensure you're comfortable and less likely to gag. If you’re really concerned, you can practice fellatio on a (washed) zucchini to get used to the sensation. You could also try different positions - for example lying on the bed with your head over the edge and throat back will make deeper thrusting more comfortable.
And don't forget to let your hands roam. Many men like their balls played with, some enjoy nipple stimulation and others, like prostate play. Vary the position to help you reach the spots that make him go 'oh!'