@Stora
Differs slightly state to state. But in most major interstates and highways, there are toll booths somewhere along the way. They have signs that have the cost as you near them. Usually there are the FastPasses (in FL at least) which are prepaid and then the regular. Regular is usually a person in a booth or a money collector. Once you give the right amount, the little line/draw post will rise and you can continue on. It's not that hard.
For crossing borders, I heard that you need to show your passport and ID... Not sure about that.
iZ