oral sex can either not feel so good for the girl or feel amazing mind blowing i guess and it takes practice and time. the best thing is to ask ur girlfriend to help tell u wat she likes. mine likes it when i move my tongue in a circular motion around her clit. also dont just start by going at it 100% start very slow, and build up pressure and speed, not to fast though or to hard big cause the clit is very sensitive.
also when ur licking her clit u can slide in a finger its best to keep ur tongue and finger at the same rhythm and if ur girlfriends ok with it slide a finger up her anus. when i do this to my girl friend she goes crazy
have fun =D remember it takes practice and patience