Your coinbase address will ALWAYS be different than any address you created through GETH.
If you created an account through GETH, it should have prompted you for a password.
It also created a private key file; usually it's located in C:\Users\USERNAME\AppData\Roaming\Geth, or something like that.
SAVE THIS FILE. IF YOU LOSE THIS FILE YOU LOSE YOUR WALLET.
I put mine on USB drives that I have put in my parent's house.
If you want to access your private wallet, you have to use a program to open the wallet. I personally use Mist.
https://github.com/ethereum/mistUse this to open your private key file with password. Then you can send funds to your Coinbase account.
Coinbase keeps all the key file + password private from you. You only have the address and the ability to send through their interface.