If you have not filed a tax return in 6 years, you should contact a tax attorney before doing anything else. An attorney can advise you on how to proceed, since both you and your father have committed tax evasion, which is highly illegal. Also, note that when you do finally file your taxes, your father will also be liable for all of the back social security and other employment taxes that he should have paid, which over six years could be a considerable amount.
In other words, this is a serious situation for both you and your father and you should not proceed without the advice of an attorney. If you come forward on your own, the IRS is usually willing to work with you and set up a payment plan. If you are caught, on the other hand, the IRS may decide to prosecute you.
So you should see an attorney as soon as possible. The longer you wait, the worse it will be if you are caught.
