Have not filed back taxes in many many years?

I have been taking care of a couple of elderly parents for many years. I bought and sold various equipment to keep afloat while caring for my parents and their daily medical problems. I haven't filed tax returns in many many many years, maybe since 1996. I can't verify anything with W-2 or 1099 or any other forms because I wasn't working FOR an employer. I have NOT been contacted by the IRS or my state. Do I just give a generic number of what I "think" I earned? I think a few companies did a 1099 form on me for $500 or so. The IRS might not have any information, but should I guess a number and hope it exceeds what I think I earned? I haven't had a normal 9-5 job in a long time because of my situation. I know a lot of people who get paid "under the table" and many who have never filed. These guys are asking me for answers now, since we've been talking about it. So do you have any idea of what will happen now that I need to back file? What information will the IRS demand? I know I need a CPA or tax attorney. But anyone with real world experience know what to expect? Everything I have read on Yahoo answers has been unhelpful. Please don't respond if you don't have specific answers. Thank you.