First game I cheated on was probably Super Mario Bros. using the level warps in 1-2 and 4-2. The first game I recall actually entering cheat codes was Doom. It's amazing that I still remember them off the top of my head, good old iddqd, idkfa/idfa, idspispopd, idchoppers, idbeholds, idbeholdl, idbeholdi, idbeholda, idbeholdv, iddt, idclip, idclev.