Or, if you are speaking to the shopkeeper or staff, you might say: 'When do you open?' For example, if you see the people in the shop putting the goods out on the shelves but it's clear that the shop isn't open to customers yet, you would ask them this question. And, as Dan Smith has already told you, you should say 'I want to' in written English!

Yes, that is a perfectly natural, correct way of saying it. By the way, you would not usually use "wanna" in written English, unless you were putting it within quotation marks as dialog.