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.
Become a dog walker if you live in an area where they are in demand. You can walk dogs for people who own dogs but who are too busy to walk them. However, this can be a demanding job, especially if you end up walking a few dogs at once or if you are walking dogs throughout the day, so ensure that you have the physical stamina for this type of job.