There are books available which place a value on collectible coins, however, to find the real market value of a specific coin, check sellers like EBay to find the prices at which the coin is is currently being traded.
I would first clean it. Then determine what state it is from and what letter of mint is on the coin. Then either consult a coin guide or a coin dealer for more information.
There are many guides writtten to determine how much a coin is worth. However, these guides can be very confusing so you should go to a coin collector you trust.