Dress light, it’s really hot in Mecca. Try to blend in and wear cultural clothing too. If you don’t have any, just buy some thobes/abbayas when you get there, they’re super cheap. You’ll be really comfortable and you won’t stand out in a crowd, which is actually much better than being obviously American and potentially catching the wrong kind of attention. Hey, it happens, just be safe. Plus, desert gear is actually perfect for…well, the desert. You’ll never be more comfortable then you will be in the local dress.
When you’re in ihram, men can only wear 2 white, un-stitched sheets…nothing else. Men can’t cover their heads, some opinions also say you’re shoes can’t have stitching either. Others say they just need to be open-toed, so it depends on who you follow. Women can wear whatever they want, but they can’t cover their faces or hands.