First you will then need to know a suitable programming language (most of them that are not the likes of php or designed for a specific use like matlab or R will do here).
Second you will have to figure out what you want to edit with your game editor. This would mean hacking the game or hoping someone else has enough data on the game for you to be able to construct an editor with it (not likely for Nintendogs).
Nintendogs is a 3d game which will make graphics and level editing somewhat more annoying but if you only want to edit text, audio, some game prices or something like that then that may not be too troubling.
We can help you with specifics but learning programming and learning hacking are things people can spend years doing, you may not have to spend years and you could have a basic but functioning editor done in fairly short order if you are lucky and willing to work a bit.