Set up account with coinbase. Go through the verifications. Get a wallet app like Bread if you use an iPhone. Buy bitcoins through coinbase. When you want to buy something send bitcoins to your private wallet (Bread) then send them on to whoever you are sending them to. Make sure your private wallet generated one time addresses for each transaction (like Bread) and that your receiver is using one time addresses as well. Follow whatever instructions your receiver gave you for verifying the transfer.
Never send or receive directly to your coinbase wallet. Always use a private mediary wallet like Bread that is not tied to you in any way.