WHAT IS THIS AND HOW DO I GET RID OF IT?

Sort of looks like ringworm, which is common in warm moist places under the armpit. Don't worry, it's not actually a worm, it's a fungus, but you need to go to the doctor so she or he can give use some topical or oral medication to get rid of it. It's highly contagious, so you should go before you give it to anyone or it spreads.