Jennifer,
When you turn the key forward, do all the dash lights dim down? If so, then you probably need to replace your starter.
If the lights don't dim, the problem is probably the fuse or relay. You can locate any needed fuse info in your owners manual. Possibly a faulty ignition switch, but that's highly unlikely.
Oh, one more thing. Please lift up the hood and look at the battery terminals. If you see any corrosion (looks like white crust) on the terminals, the current from the battery to the alternator will be hindered. To remedy this, put baking soda on the terminals. Then pour regular Mountain Dew on the baking soda. You'll see alot of foaming, but that's just the corrosion gettin' ate up :-) Rinse with some water.
Good Luck,
Liddy