When you have finished picking stuff out, the site will ask you how you want the order shipped to you. The shipping costs will be posted next to the various methods, along with their expected delivery time. You should check the site's return policy before (not after!) you order something. There's a pretty big difference. Some make you pay the shipping, some don't. Etc.
Shipping generally takes about a week, Shipping can be up to $10 or $15 in certain stores, unless you find a special for shipping going on. For returns, you can take them to the actual store or mail them back.
Okay. When you check out it will tell you how much shipping costs and how long it will be. And if it isn't the right size you just send it back and order a smaller/bigger size. Just ask your mom to help you. And your probably too young to be shopping at Urban Outfitters anyway.