Fuel filter: It’s about $5 on 12A-powered 1st gens (ie: those with a carbureter rather than fuel injection) and located under the car on the left side just ahead of the fuel tank. Unless the person who sold you the car can produce a recent receipt saying this is a new fuel filter it’s likely time to change it. It’s a simple matter of pinching off the fuel hose leading to and from the filter with a couple of vise grips or clamps, then removing the hose clamps that hold the filter in place and finally removing the filter itself. Install the new filter, making sure the direction of flow indicated on the filter body is correct with the flow of fuel from the tank.
Air filter: If you can’t see light through it, or see opaque dark clumps, it’s time to change it out. Clogged filters cause excessive fuel consumption and lead to poor engine performance.
Oil Filter: see oil.