If you bought it on your Amazon account it is already registered.
If you are registering for the first time then you just use your normal email and password you use for Amazon every day.
If you have bought a second hand Kindle then the previous owner needs to de-register it. If they have not done it then you may need to log into the amazon site on your pc, call their help desk and give them the FIre serial number. They should be able to force a re-set assuming it has not been lost/stolen, in which case they may have locked it and you have a paperweight :(
Answers & Comments
Guessing you can connect to wifi
If you bought it on your Amazon account it is already registered.
If you are registering for the first time then you just use your normal email and password you use for Amazon every day.
If you have bought a second hand Kindle then the previous owner needs to de-register it. If they have not done it then you may need to log into the amazon site on your pc, call their help desk and give them the FIre serial number. They should be able to force a re-set assuming it has not been lost/stolen, in which case they may have locked it and you have a paperweight :(