As a guy who has both given and received blowjobs, I think it is pretty straighforward. Did he keep his erection and did he ejaculate? If so then you were right on, if not, then he has some problems. There is not a lot that can be done wrong when sucking on a penis, teeth is one and changing your rythym when he's near orgasm is about all I can think of. And believe me, there is no real way to practice giving a blowjob before hand. It's like, okay I've got his penis in my mouth...let's see what I can do with it and what I am capable of.
Don't doubt yourself, or be hard on yourself. You already did him a great service by just putting it in your mouth.