Okay, these kinds of things are quite common on the shaft of the penis. The thing to try first is to put liquid (not cream or ointment versions) antiseptic on it. If you live in the UK get yourself some TCP - you need to dab a little directly onto this 4 to 5 times a day, it should start to reduce in size after 3 or 4 days when you can reduce application to 2 or 3 times a day. Apply 1 or 2 times a day until it is completely healed.
Should it not heal seek professional medical help.
If you are sexually active, particularly with new or different partners you need to use condoms until this is healed as you may be more susceptible to infection …