A roth is a great place to have your money. FYI you are generally limited to 5k/year that you can contribute toward it. It doesn't affect your taxes the year you contribute but once you put it in there it can now grow tax-free, and best of all the original contribution can often be withdrawn early without penalty later if you run into an emergency. If you have no taxable income then it's a great time to put money into a roth, I would even consider it instead of paying down debt if you have no "taxable income", but likely you do have taxable income left even after claiming your girlfriend and her kid. If you have credit card debt that you can't easily pay off within the near future then you'd probably be better served financially to pay it off. However, these are just general thoughts and so much depends on your specific circumstances. I would advise you to consult a tax professional.
The contributions will likely make no difference whatsoever to your taxes. In can make a very small difference to your bottom line in some circumstances but usually only if you own a home and itemize deductions.
The federal loans someone mentioned. I will guarantee you that they will eventually grab your refund if you don't get them out of default. It's just a question of when and if you've gotten away with it for four years, it will probably be soon.
