I wouldn't depend solely on commissions, don't start this then quit the regular job; I'm doing that right now as a student and freelance and the flux of income is so erratic and undependable that I really don't suggest it as your sole income, especially since this would be your first time doing commissions and it can take as long as a few months to really get out there. As far as how much you should charge: That really depends on what you would feel comfortable charging. Maybe do up an example sheet, slap some prices on there, test the waters. As for getting yourself out there, there are a lot of things you could do. I got my start doing freebies then slowly transitioning into paying territory, though I don't really recommend that route since it's super tiring... A lot of it is worth of mouth, unless you feel like shelling out $20 for a moth of ad space on here.

sorry if this is one long pessimistic comment, lol, just trying to be realistic about it.

I agree with Toxius, you really need to get your name out there in order to even try to make any money doing art on here. Freebies are a good idea and YCH also seem to do good for trying to get out there. Try to find artists that you feel are about your level and look to see what they charge for art and base your prices loosely off of them. Personally I'd love to see you start opening for commissions but sadly its a slow start.