In order to determine the interest rate, a government's economic observers create a policy that helps ensure stable prices and liquidity. This policy is routinely checked so that the supply of money within the economy is neither too large (causing prices to increase) nor too small (causing prices to decrease). In the U.S., interest rates are determined by the Federal Open Market Committee, which consists of the seven governors of the Federal Reserve Board and five Federal Reserve Bank presidents. The FOMC meets eight times a year to determine the near-term direction of monetary policy and interest rates.
For further background on the Federal Reserve's functions, see our tutorial on The Federal Reserve.
Retail banks are typically the first financial institutions to expose money to the economy, and therefore they are the principal instruments used by the central bank to manipulate the money supply. Put simply, the central bank is able to regulate the supply of money to the end user (individuals and companies) by adjusting interest rates on the money it lends to or borrows from retail banks.
If the monetary policy makers wish to decrease the money supply, they will increase the interest rate, which makes it more attractive to deposit funds and reduce borrowing from the central bank. Conversely, if the central bank wishes to increase the money supply, they will decrease the interest rate, which makes it more attractive to borrow and spend money.