How do i raise the handlebar on a 1999 MGX bicycle?

You can only raise a quill stem os far safely. Loosen the vertical center bolt that's in line with the steerer tube a few turns and then tap down on it with a block of wood or rubber mallet to unseat it. The stem should be loose enough to turn independently of the front wheel. You can also raise it somewhat, but look carefully for the "maximum height" or equivalent markings etched into the stem body. Don't expose any more stem than up to that line. Line up your handlebars with the front wheel and securely tighten that bolt. It should be tight enough to vigorously resist independent rotation when you hold the front wheel bewteen your knees and attempt to turn the handlebars.

If it's a new-style threadless type stem, the maximum height was determined when they assembled the bike because they had to cut the steerer tube to be below the top edge of the stem. Go to a Bike Shop and have them replace the stem with one that has a higher rise or shorter reach.