EDIT: For solo mining you need your Ethereum wallet running and synchronized.

For pool mining you need to specify the HTTP server and port. You also need a worker's name and address.

For pool mining it would look similar to this: ethminer -G -F http://eth-us.dwarfpool.com:80/0x040fc5e5ac72edb8b27dcc19ba56176e2b4fb0ac/ub0
or this: ethminer -G -F http://eth-mine.suprnova.cc:3000/Greased_Lightning.rig1/30
  
ethminer -G -F http://eth-mine.suprnova.cc:3000/Weblogin.Worker/YourHashrateInMH/s