I would suggest a small knowledge of a photoshop like application, such as GIMP, a texture template (viewtopic.php?f=25&t=60625), a general idea of what you want, and the internet, to upload it when its all said and done.
screw paint.net it doesn't transparent ok its simple you get adobe photoshop because it stays transparent you take the mobs and terrain png and done
Photoshop is extremely expensive, dude... Unless you were to pirate it. But that's beside the point. Also, you've clearly never used Paint.net and are clearly thinking of MS Paint, as Paint.net does in fact have transparency.
To make the textures, first, you have to get some kind of image editing program that supports transparency (like Paint.net) and any program that can turn files into ZIP files (like WinRAR). Then, you have to open up your .minecraft (just press the [open texture pack folder] button in your texture packs section of the client. Next, open the bin folder, right-click MineCraft.jar and select open with WinRAR (if that's the program you have). You should now have a folder with all of the images and codes needed for the game. Take all the images, including the folders they're in and place 'em in a new folder on your desktop. This new folder should have the name of your texture pack. Next, just change the folder into a ZIP folder (this part, I'm not 100% sure how to do) and there you have it! All you need to do now is edit the images! :happy.gif: