Clean it up, lube, get the saddle and bars adjusted and ride it! See how it does. Check the chain for stretch, replace if necessary. Can the wheels spin without rubbing on the brakes? What shape are the brake pads in? I'm guessing it probably has cantilever brakes, if so and they are the long ones do you have any heel clearance issues? Resist the urge to upgrade anything that isn't broken or worn out. Steel rigid mtb's are fun and will make you a good rider.