Suggest Treatment For Rashes On Penis

i am a 19 year old male. I have only had sex with one girl, who was clean. I started to develop a rash on my penis a few days ago and now the underside of my penis is starting to swell. i have no problem urinating or getting an erection, it just itches alot. i dont want to go to the doctor so is there anything i can do to make it go away without going to the doctor?