At the risk of acting like a total nOOb…..

I just bought a nintendo DS. I purchased nintendogs used. I need to find out how to clear out the previous users saved progress. I have tried everything I can think of. If anyone can help I would be very thankful.

Should you ever wish to reset ALL game data (owner information, items,
puppies – EVERYTHING), you can do so by starting up the DS, touching
the Nintendogs startup bar, and then QUICKLY hold down L, R, A, B, X,
and Y. The game should now ask "Reset all game data?" Tell it yes,
then wait a few minutes as it deletes all the data (DON’T turn it off
while it’s doing this – it says so itself this could screw things up).
You’ll now have a blank cartridge just as it was the day it came out
of the box, ready to be played like new.

From some random GameFAQs :3

MOG you are my KNIGHT. I LOVE YOU!