I Need help so badly ive been trying to fix this for weeks on end please help me

I bought an iphone 3g and it wont restore in iTunes am getting all errors and phone stuck in connect to iTunes logo on bootup as I cant boot it up and when I go onto iTunes it says Serial Number: N/A and nothing will restore as I have no backups but when I download a 3gs backup and try to restore I get errors all time and the SIM Slot is registered to a 3GS on IMEI checker please can someone help me get this phone to work so I can use it thank you for your help guys