What program would I use to make a Texture Pack?

gimp
or paint.net

Aseprite is a cool one :D

Mobs and any other 1:1 or 1:2 ratio texture can be edited here: https://www.planetminecraft.com/pmcskin3d/

For blocks and items you will need to use 2d mode.

Hi, as they already recommended you you can try Paint.NET, but if you want more advanced tools for photorealistic texturepacks you can use Photoshop.

I recommend Paint.net if you'd like a free program that can be made to be almost just like Photoshop if you download a few plug-in packs from its forums. If you want that but with a more complex and irritating GUI, get GIMP. If you don't want to download a program, you can use Pixlr E, which is an online photo editor with many of the same useful tools, and most of them work just as well.