You can't really 'create' a Game Genie code. You actually discover it, because all the codes are already created. You just have to find out which codes do crazy things and which ones don't. So here we go (SMB1 for NES):

 

ATPAUY - Mario doesn't earn any points!

LLPOKG - start on world 4-2, warp to impossible to beat world

YYPOKG - start on underwater 1-2

KTIPEK - Black and white world except for pipes and trees are orange. Mario walks until he reaches a pipe and is stuck.

IIPOPK - Brick floor with no background

PSILUV - Once you hit a ? box with a coin in it, the box disappears!

NNINTV - Beep starts each level. Land music is the same, but no music plays in caves, and a high-pitched never-ending beep in the water worlds.

NATUTV - Slow Mario, jumping on enemies kills you!

STYYLY - 1-1 has weird music. At the end of level 1-1, end music plays, game freezes.

_TPAUL - Fill in space with different letters and you get a variety of different squares as your background.

POEYPO - Mario is invincible, but you can't see him!

YEAAAA - This makes the stuff that happens in the PIGPOG code look normal. We're talking weird stuff, background color changes, numbers flooding the screen, making the game slow down a LOT. If at first, it freezes after a little bit, try it again because the stuff it does is random.

GAYSUK - On a ledge, get a running start and you can run off the ledge in mid-air!

 

I don't think these codes are anywhere else, so there. I can always try & discover more if you people like these.