There is an update.zip on my SD but I think that's the ROM Manager JT Rom I tried to flash after flashing CWM via Rom Manager.
-> Did you succeed flashing it? what happened after you tried to flash?
"E:failed to verify whole-life signature
E:signature verification failed
Installation aborted."
-> This usually happens when odin didn't flash (kernel and recovery partition) CWMrecovery.tar yet. Odin has to enable update.zip on your sd card in order to flash it.
Which I know is a semi common problem. Should I delete that update.zip and try again? Do you know how I could completely unflash CWM and start over? Or do you have any other tips?
-> if "E:failed to verify whole-life signature
E:signature verification failed
Installation aborted." messages showing, CWMRecovery didn't flash anything to your phone. Unroot your phone(if there hasn't anything done as far as rom or kernel installations; if not you need to remove them), factory reset, format your sd card, and start all over.