I can think of two low level ways to communicate with a circuit - digital (either something is on or off) or analog (what voltage is at specific point). To acomplish these tasks you need a microcontroller. Microcontroller is a very small computer on a chip that has means to communicate to circuit and to humans.
As i understand, you are not experienced in electronics very much, so i'd suggest an Arduino board - it's a board with microcontroller and other stuff that makes it easy to use.
There are some FTDI chips as suggested by justin, that can do digital communication too, there are loads of other microcontrollers and development boards, but arduino in general is the most friendly to beginners.