bad alternator probably caused the light to come on, or faulty/loose wiring coming from the alternator. to replace spark plugs. you need to remove the spar plug wires, and then with the correct size spark plug socket you unscrew the plugs from the head. before you install the new plugs you need to check the spark plug gap (gap specification is under the hood on the engine spec tag). coat the spark plugs with antisieze (if you dont you'll be sorry next time you change them).