CAS is all the manipulation of the "create a sim" tool within the game; this section covers how to edit parts of the program and its tools using different programs such as the CTU, it also outlines tools that are useful and/or vital for the tutorials. All the tutorials below have been split into specific categories.
Paint.NET is one of a couple of programs that can be used to edit Sims 3 textures. Get it for free, here.
GIMP is another - free - program that can be used to edit Sims 3 texture files. Find it here.
Photoshop is, perhaps, the most used paint program. However, it can be very expensive. The bright-side is, a lot of schools/universities/jobs offer free or discounted versions of Photoshop to their students/employees.
DDS Plug-Ins and Converters
In order to get your textures to open in your paint program, you will need to install the proper DDS plugin for it or convert them.