Single sex secondary school to a mixed sixth form

Hi im an average looking guy from the UK, Im 16 and I go to an all boys secondary school. My problem is i'm in year 11 and soon I will be going to sixth form (Year 12/13), however the sixth form is mixed. I'm straight but I havent properly talked to a girl since primary school. I dont no how to act around girls but I would like to know how to in a way that could get me into a relationship, or even be friends with a girl. I'm a really nervous guy and wouldnt even no what to say when even standing next to a girl. All my friends that are in sexual relationships say that "You need to be confident.", but thats the problem...I dont know how. Any advise would be most appreciated as it wont be long untill I go to sixth form. Thanks :)