This is a list of software you can download for free to  start sketching and developing your games:

Autodesk Sketchbook Pro

It’s free for students, the license is for 3 years I think, you will find tutorials on the students site.

Unit 40 Information sketching your designs,
Autodesk students website screenshot

KRITA Free digital painting software (Open Source), my advice is that you will need a tablet (graphic) to work with it. It’s a bit difficult to understand at first, but once you grab an understanding of the principals and tools you good to go coming up with good designs.

Unit 40 Information sketching your designs,
Krita home page screenshot

Krita team also gives you the opportunity to support the development fixing issues with the software. It’s a good opportunity for software dev and programming students.

Unit 40 Information sketching your designs,
Krita development opportunity. I think IT students should take the opportunity

Autodesk Sketchbook Pro