I'm not sure what you're asking exactly, I never heard of Aquaria saying "create mymod".
However, to play/edit a mod you'll need an .xml file with the same name as the folder of your mod, with some details in it, look at the other .xml's to figure out what.
Then you need a .png file which is your map layout, and that's the minimum, if you want to add things you'll need to add a whole lot more.
I suggest you start looking at the modding tutorial first, that's where I started.