2001 Ford Escort Service Engine Soon light is on

My question is about my 2001 Ford Escort. After I put new spark plugs and wires on the Service Engine Soon light came on about a week later. It drove fine so I have ignored it. I needed brakes put on and decided to have them also check about the light. They said I needed a PCV valve and hose. I also got them to replace the fuel filter and clean the system. After I picked up the car, the Service Engine Soon light did not come on, however, after starting it 2 more times while running errands, the light came back on. Why would this happen? Can I reset it myself or does the dealership where I had the repairs done need to reset it? I would appreciate your help with this question. Thanks.
Debbie M.