Hey Jyris,
First off, if this is your first time here, I would recommend checking out the main melee hacking thread
here, and also the official melee texture hacking thread
here.
Now, as far as creating a texture for Jigglypuff is concerned, you're mainly going to be working with .dat Texture Wizard. It's an extremely convenient and intuitive program made by our very own DRGN. You can find that program
here.
Aside from DTW, you don't really need much other than a Melee ISO, an image editor, and a dream.
If you have any more questions about the exact process of editing textures, try to keep them on the texture section for the Melee Workshop
here. But before you ask questions, make sure you read through the official melee texture hacking thread and the thread for DTW to make sure you're not asking questions that have already been answered!