How To Treat A Red Rash On The Penis?

Hello. I am an 18 year old male. I have never had sex but I I've had sexual experiences. Anyways. For as long as I can remember I've had a very itchy red rash at the base of my penis. It has never really come to mind but now that I have a girlfriend for quite awhile I feel we are going to have sex (protected). While I'm fairly certain it's not an STD it is cosmetically ugly. What is it/what can i do about it?