Most casinos have a cashier's cage or a designated area where you can exchange your chip for cash. Make sure to bring a valid ID with you when you go to cash it in. If you're unsure about the specific procedures at the casino, you can always call ahead and ask about their policies for cashing in chips.
In general, the older the chip, the more valuable it is. A textbook example that proves the point is a circa-1948-51 Club HarlemR$5 chip from the Reno, Nevada casino of the same name.