I highly recommend at least trying out the free demo of Art Rage; I use the program myself for quick sketching and underlays, such as in the quick video tutorial above and the sketch below. The tools are very high quality and the realistic paper texture is nice to work on.

Unfortunately you can't use layers in the demo. It's enough however to give you a really good feel for the full program which is very reasonably priced if you do want the extra features.

I am often asked what equipment I use and what I recommend for digital work. Firstly I recommend the best Wacom tablet you can get. If you can get a Cintiq, that's ideal but you don't need one to produce good work. If you can only afford small and basic, it's still the way forward. My first Wacom was the smallest you could buy but with it I learned the skill of using a tablet . In terms of software, everybody has their preference but Art Rage (free demo available, link below) and Sketchbook Pro are good, affordable places to start. I also recommend using French curves with your Wacom for more control over your linework. Here are some products to give you an idea of cost.