How Can An Armpit Rash With Swelling And Itching Be Treated?

i have had an ongoing rash for months to a year now of a dark red rash in my armpit it only happens when i dont shave my armpit and sometimes it gets swollen and itches and dries out today i popped a black head/ ingrown hair and the pus that followed was green...also a cousin of mine had a similar problem can you help?