I recently upgraded iMac. I used the migration assistant to transfer info from old iMac and it transferred everything however it created a second user ID, how can I combine the two accounts so everything is in the same user?
That is the problem with Migration Asst - it will result in having two accounts and can take a while to sort out.
If you use Setup Assistant (at the beginning of the setup process) and have the firewire (incl. a Thunderbolt adapter if necessary) ready to connect the two Macs, the Setup Asst will port over your account as is and you will wind up with just one account. So starting over may be easier at this point.
No, you need firewire (if your Mac has a FW port) - see some info here under setup:
To go back to the startup menu, you can use recovery - see info here:
You'd basically start with key commands Command + Option + R, choose Disk Utility to erase the drive and then Apple's servers will reinstall the OS. Once that is done, the setup will begin and Setup Asst will be offered in the beginning (it's only offered once - you decline and you have to use Migration Asst or start over).