What should I know when buying a MLS Jersey?

Hi, I'm want to buy a MLS Jersey but I don't have any Major stadiums near me so I'm going to buy online. (ebay i think) I want to know what the actual players ware not some cheap white T-shirt that has a logo on it. It seems like they ware some Polyester or something? but what is it? and what size should a 13 year old get (average weight and height) ??? Please Help?