Dark underarms is a problem for almost everyone, so you're not alone! Dark underarms are caused by dead skin cells that form underneath your arms, so the best way to get rid of them is to exfoliate with a body scrub every day. This will remove all the dirt and skin, so you will have cleaner underarms. You can also try using a underarm cream, or lemon. I'm not sure if lemon works, but I have heard that it comes in handy sometimes. The last tip is, instead of shaving, try plucking or waxing. When you shave, it cuts the hair but leaves the dark roots of the hair inside your skin still, which leaves the appearance of dark underarms. If you pull out the hair all the way, there won't be anything dark hiding underneath. I hope this helped!