You could try getting good with a mouse, but it'll be less efficient, more difficult and ultimately pointless compared to buying a tablet. You can get one pretty cheap, my Bamboo Splash was about £38, and it's very good, it's probably one of the best ones a hobbyist can get. I mean you could learn to draw with a mouse, but it's just not worth how much more difficult it'll be.
Also don't use Paint.net with a tablet. That's stupid advice. Paint.net doesn't smooth out your lines like Paint Tool SAI does.
Paint Tool SAI vs Paint.net
Paint Tool SAI is less confusing than Photoshop, but if you're willing to learn how to use Photoshop, go with that. Me, I just draw in SAI and touch up in Paint.net