Just go for it. If your bf has had head before, then he'll know what he likes.
My bf just places his hands lightly on my neck, and tells me what to do. Such as "suck more", "go deeper" etc.
If it doesn't feel right, don't do it. If you end up giving him head, it would most likely be during foreplay, and even if you are bad, your bf will know you're inexperienced anyway and won't care.
It's hard to give a guy bad oral, all you do is suck his cock, not really any more or less, just the technique I suppose. But ye, make sure you don't use teeth... I do but my partner likes it