Buick LeSabre Problems Answered

2003 Buick Lesabre 3.8L Check Engine Soon alone with Gas Cap Loose. Both have been on for about three years.
Question: My 2003 Buick Lesabre is making noise. I thought it sounded like a wheel bearing. This was confirmed by a mechanic at a Firestone store. What I'd like to know is how do I determine which wheel it is and how difficult is it to change or should i get a mechanic to fix the car?