First, what iPhone do you have, 2G, 3G, 3GS or IP4?
If it's 2G or 3G, you can easy find custom firmware and how to restore by Google for them.
If it's a 3GS or IP4 and you updated to official firmware, you won't be able to install any custom firmware because your phone is no longer pwned. Your only course of action is to downgrade to previous version if you have your SHSH saved. Google: downgrade + SHSH if you don't know how