If it has find my iPod enabled on it, you'll have to remove it from your old iCloud account first to be able to use it.
To hard reset it: you'll want to enter
Recovery Mode, and download the latest
SIGNED firmware for your device.
http://ipsw.me is great for this. Go into iTunes, and click restore. Select your IPSW file, and wait for it to finish restoring, and you'll have a clean phone. You CANNOT set it up without removing it from your old iCloud account, however, due to Find my iPod and etc.