It depends on your age. If you are under 16 you have a children's account, which does not allow online use. If you are 16 or 17 you should have a young person'r account which does permit online use (as long as you upgraded from a children's account when you reached 16).
Yes. Just be careful who you are paying and check the address to make sure it's real.
if its a Visa/Mastercard debit card, sure
Yes you can in the UK
