If you have to pay it now, you have to take out a loan, ask for an advance on your salary or borrow money from friends or family. If it can wait, the amount will accumulate interest until you pay it back. If it's just one month out of twelve, and the other eleven you always have money left over, there's not really anything to worry about. Put the left over money aside to pay this month's expenses. Other than that, there are only two things you can do: spend less, or earn more. Sell your car, move to a smaller place, get a roommate. Cut whatever expenses you need to break even. Try for a second job, a raise, or sell something you make.
Try flipping houses if you have experience with making home repairs. As you may know from watching popular home improvement shows, flipping homes involves buying up a lower valued property that needs work, and then fixing it up for resell. To get started, you’ll need to have financing either through a bank a partner. Then, you can buy a property that’s priced below market value. After you renovate the property, you may be able to sell it for a profit.