Probably the easiest way is to pay online, using the Electronic Federal Tax Payment System that's available here:
https://www.eftps.gov/eftps/direct/EftpsHome.page
At this site you can schedule future payments as well as any payments that you want to make currently.
The US tax system is a "pay as you go" system and most folks "pay as they go" by having taxes withheld from their paychecks. If you have significant income that's not subject to withholding, e.g., independent contractor income, you may have to pay "estimated taxes" directly to the treasury. Generally these taxes are due more or less quarterly around 4/15, 6/15, 9/15 and 1/15 of the following year. I emphasized the word "may" because it's not exactly a hard and fast rule. It's perfectly OK to owe the government a lot of money when you send in your income tax return, but if you're "underpaid" - a piece of tax jargon with a definite meaning - then you could be subject to underpayment penalties.
Look at the "General Rule" at the site Kat has provided and the "Special Rules" and you may find out you don't really need to make an estimated tax payment, depending on your personal situation.
