I have a constant rash on my penis what kind of doctor do I see

I would go to your family doctor first and then go from there. If the doctor thinks it is worth seeing a specialist, you will be given a referral. If you don't have insurance, see if there are any clinics in your area which bill on a sliding scale based on your income. The doctor will want to know how long it's been there, does it hurt, etc. I think you should get it looked at right away. They will probably do STD testing (if you are sexually active).

A dermatologist

Go to a walk in clinic and they should be able to diagnose, treat, or refer you.