So assuming that you can't full pay your liability, you have a couple of different choices. First if you only need a temporary solution, you can request the IRS give you up to a hundred and twenty day extension to pay your taxes. The IRS will usually do so with little fuss. If you need more than that, the IRS is going to mandate that you get on a payment plan:

what we call an installment agreement. The IRS is going to want to see measurable progress towards paying off your liabilities based on your financial circumstances and the amount that you owe. If you can't afford to pay your taxes, then the IRS will consider placing you on what's called currently not collectible status. Currently not collectible status is a temporary hardship status that the IRS has for certain taxpayers. Generally currently non collectible is a temporary status. It will last up to two years at which point the IRS will review your file and may request updated financial information to keep you on currently not collectible status. The next option you have is working on a tax settlement with the IRS which is called an offer in compromise. You make a promise to pay and pay your taxes and file them on time for a five-year period. Going forward in exchange for that and an additional lump sum, the IRS will forgive your past tax liability. Finally if none of the administrative remedies work, in the IRS you always have the option of filing for bankruptcy. Taxes and bankruptcy have to meet certain requirements and you do have to go through the bankruptcy process, which is a separate issue in and of itself but it's also a way to deal with your taxes. So those are the ways you deal with taxes when you owe more to the government than you can afford.