I have a Bitcoin Core wallet. I do not know what address is required under 'Pay to'. Is it one I must create or what?
This should be an address belonging to the person or company you want to pay bitcoins to. You have to get the address from them.
If you are trying to buy something from a company, usually they will tell you the payment address as part of the purchasing process. If you are trying to send money to a friend, ask them for their address.
If you are not looking to pay money to anyone right now, then you do not need to use this screen at all.