Please help know how to behave around guys?

I've not been in co-ed since 16, now I'm 22, and I'm joining post graduation, I ve not got even the slightest idea how to treat guys, how to communicate with them, esp how not to behave with them, what not to tell them. it makes me nervous. can u please list me what not to do in a co-ed and how to treat guys in classrooms?makes me nervous. can u please list me what not to do in a co-ed and how to treat guys in classrooms?