Usually, it’s capital that you need. But sometimes, and specifically right now if you’re in the United States, capitol’s the word you’re looking for.
Capital is obviously the more common word, and one of the most common contexts you’ll use it in is to refer to the capital city of a country or state.
Capitol though, is defined as a building hosting a legaslative assembly, as used largly in American English.
Both words are derived from the Latin caput, meaning head, though capitol more specifically comes from Capitolium, a temple to Jupiter on Ancient Rome’s Capitoline Hill.
The two are understandably confusing, especially when talking about American politics. You could refer to the capital city of a state as the state capital. But the state capitol is also the building in which the state legislature sits. And of course, you’ll usually find the state capitol in the state capital.
And that’s not even mentioning the Capitol (capitalised!), which is located in Washington D.C. and is the seat of the United States Congress.
No wonder it’s confusing!