Aha! A capital is a stash of money or the government headquarters of a state. Oh, a capitol is a building.

A state's capital is the primary city and usually the seat of the state government. The most important city is the capital city. The capital of New York is Albany, for example. Washington DC is the nation's capital:

Park Police found an abandoned vehicle, with an assault rifle inside it, near a bridge leading out of the nation's capital to Virginia. (New York Times)

Capital is also a financial word referring to, in a nutshell, the money a person or business has, not including debts. But wait, there's more! A capital letter starts a sentence or a proper noun, like the capital V for Virginia. It's also a crime that carries with it the severest penalty (life in prison or execution):

Wilson is the 19th death row inmate to commit suicide since California reinstated capital punishment in 1978, according to the Department of Corrections. (Reuters)

If you want to sound like a British spy, use capital to mean excellent, first rate, or really important:

In the realm of amateur fruit growing, on the other hand — a realm now daily widening — dwarf fruit trees are of capital importance. (F.A. Waugh)

A capitol (with an o) is a building that houses a government's legislative branch. When capitol is capitalized (ahem), it refers to the Capitol Building in Washington, DC, that is home to US Senators and Representatives. It's located on — where else? — Capitol Hill.

Remember that capitol with an o refers to a certain type of building that usually has a dome, then you'll know that in all other cases you want capital with an a.