For 2009 & 2010, the IRS files (based on the W2s and 1099s they received) whatever the highest possible liability could be for the years they assess. This is done in some part to motivate you to actually file the taxes. When you actually file these taxes, in many situations (but not all) you may actually owe less than the unfiled / unassessed taxes. The best advice is to get your taxes filed and to continue to file every year moving forward, preferably with a tax professional's help. Best of luck.
$6,900 is what the IRS says you owe them based on the information available to them. That may or may not be true, and nobody here will be able to tell you without a lot more information.
You can stop the IRS from garnishing your wages, but to do so, you'll probably need to agree to pay at least some of the back taxes one way or another. First you should file returns for the missing years and hopefully lower the underlying tax owed. You might end up owing nothing, or owing more. I really can't say at this point.
After filing returns, you should weigh your options and, depending on your financial situation, try to enter an Installment Agreement (a monthly payment plan), make an Offer in Compromise (settle the debt for less than the amount owed), have the IRS declare the debt uncollectible, or pay in full. I have no way to predict what options will work for you. Speak to a licensed tax professional and discuss your situation in more detail to be certain of what options are available.
Robert Hoffman is a tax attorney licensed in California. The information presented here is general in nature and is not intended as a substitute for legal advice. This posting does not create any attorney-client relationship with the author. For competent advice about your particular situation, consult your own attorney.