There are several different reasons why your car's air bag light may come on.
1. There is something stuck in the seat belt buckle that is interfering with the seat belt switch sensor. This sensor detects whether or not the driver's seat belt is properly fastened. In some cases, if something like a coin is stuck down inside the buckle, it can cause a false signal to the air bag light, triggering it to come on. You can either check your seat belt buckle yourself, or take it to a mechanic to do it for you. 
2. The light could be on because of a damaged clockspring. This is an electrical connector that keeps the steering wheel's electrical system, air bag and horn connected even as the wheel turns. If the clockspring is broken, the air bag won't be able to deploy. 
3. The wires under the seat may have gotten damaged if someone shoved something up under the seat.
4. If the car was in a relatively mild accident that was strong enough to trigger the crash sensors, but not strong enough to deploy the air bags, the air bags may need to be reset. 
Be aware that if the air bag light is on, it means that the air bags won't deploy. Because of this, it is important to solve the problem as quickly as possible. Otherwise your risk of serious injury in an accident significantly increases.
This video does a great job of explaining how to identify the code that your air bag light is giving you if it flashes after you start your vehicle. If the light just stays solid and doesn't flash, you will need to take it to a mechanic or auto parts store to have the code read. Usually code reading is done for free.