Looking at my engine I can see a belt. i can also see something sort of under the belt that lies flat against the engine and it moves. what is the name of the part moving when i turn the AC on? I have a 98 Mercury Sable V6 engine.

First make sure it is the correct belt for that engine. Sometimes the really cheap belts are often not quite the correct size so they wear out faster. If it has covers over the belts are they fitted correctly? Are you having any other issues with the car? A battery not charging or an over heating problem? These can give clues to possible reasons for the belt wear. Where on the belt are the wear marks showing? If it is on the smooth back side of the belt there is probably something rubbing on it. Look to see if there is something else in the engine bay that is sticking out and touching where it shouldn't.
If it is on the ribbed, working side of the belt it could be a pulley wheel causing the issue. The tensioner could be sticking or failing so it isn't adjusting for engine speed changes. It could also be one of the pulleys it runs around 'dragging'.
With the engine OFF remove the belt and check the tensioner moves freely against the spring inside it. Turn all the smaller pulley wheels the belt runs around by hand. They should turn smoothly, freely and quietly. If they feel 'gritty' or seem noisy that could be where the issue is.

No the belt should should not rub on anything. Go to a ford dealership and ask the parts counter guy for a diagram of the belt routing they should print you off a copy no problem. all the ones i go to are happy to assist

theres a button under the dash.. if you lie down and look under the dash, push the brake pedal with your hand.. you will see it press against a button, that button is bad.. when the brake pedal pushes against that, it unlocks the shift selector so you can choose your gear.. change that and your problem will be solved.

http://www.clubfte.com/users/racerguy/zetecbelt.gif

Hope that helps you.

You don't need that. All you have to do is use the 'under over' method.
You see, the pulleys are designed to ride on the 2 different sides of the serpentine belt.

Simply put the flat ungrooved side against the flat pulleys, and put the grooved side on the grooved side of the next pulley, until you've done all but one, the idler or tensioner pulley.

Use a breaker bar to move the tensioner pulley in a way that the serpentine belt can be slipped on.

And that's it.

I am not familar with your particular vehicle so what I provide is general guidance. I am addressing this for a serpentine belt (not to be confused with a timing belt). If you removed any of the pulleys in replacing the belt check and make sure you put the pulley on correctly. It is not that difficult to reverse a pulley when putting it on. If you losened a pulley or tensioner then go back to those pulleys and make sure the pulley is lying flat against its mount. Take a look at which pulley is out of allignment compared to the others. The one pulley that is out of allignment is where your problem is. If you did not remove or losen any pulleys (only losened a tensioner) in pulling the belt just go ahead and put the belt back on and see what happens. If it was a timing belt you were talking about you would never just try putting it on as poping a serpentine belt is not a big deal but pop a timing belt can ruing your engine.

What you have to do is make sure you have a new belt. Then look under the hood, you don't give a year or engine size so some of these directions may or may not apply. Check to see if the alternator belt is the outside belt or if others need to come off first. If needed remove belts that are in the way taking note of routing. If its one serpentine belt take note of routing (there should be a sticker under the hood with routing). Remove belt by loosening the pivot or tensioner. Then replace and set tension. With tensioners the tension is set automatically. If you are not sure what a tensioner is look at the belt, you may want to push or pull on it and you will see a pulley that goes to nothing but is against the belt and will move when you move the belt. To make it easier to remove belt look inside the tensioner pulley and there is a bolt put a socket on it and move as needed to take pressure off belt and remove belt. Take your time and look at the belt(s) before starting. Take notes or draw a picture to help putting it back together. Belts just look hard but are easier than they look.

You may be looking at the heat shield above the exhaust pipe. For the most part this is not an expensive fix unless you live in the socalled "saltbelt" where the salt from the snow removal eats away the mounting points on the shield.

I don't know specific cars. But usually there is an adjustable pulley. You loosen the nut in the center of the pulley wheel and then there is usually a bolt you tighten to pull the pulley wheel down to tighten the belt. Then you tighten the nut in the center of the pulley. I check the belt tension by pulling or pushing the belt down on the backside non ribbed side almost as hard as I can (with only index finger) and see if belt moves about 1/3 inch for belts freespan between round pulleys, or alternator, or whatever less than 12 inches. If the belt freespan between pulleys is more than 12 inches I look for the belt to move about 1/2 inch. Also thin belts I allow more deflection. Also push or pull on belt in middle between round things it goes around, with finger.
If there is no adjustable bolt to move pulley then there is usually a lock bolt and an adjustable bolt. The lock bolt sits in a crescent shaped slot. You loosen the bolt and usually pry the alternator or whatever with a big screwdriver or pry bar. You must usually pry against the engine against the alternator but not on something easily broken or bent, like an oil pan. You want to pry against solid metal. If it doesn't seem to move you might have to loosen the pivot bolt. It will usually be at the center of the slot visuallized as part of a circle. Look for the center of the circle and loosen the bolt or bolt and nut. And don't forget to retighten it.
The third kind of belt it could be is the serpentine belt. Usually a long belt, usually only one which drives everything like power steering, alternator, water pump, AC compressor, etc. There is usually a wheeled tensioner you have to loosen with a 3/8 or 1/2 breaker bar or ratchet, or sometimes a wrench. Usually there is a square hole to fit the ratchet that you try to rotate in one direction to remove tension on the belt. I try to put moderate pressure in both directions, clockwise and counter clockwise. It should move and increase in the force you have to apply in one direction and feel like it is hitting a stop in the other direction. Turn it into the part that feels like you are pushing against a spring, which you are,
then rotate as far as possible that way. Then while holding ratchet have someone remove belt and put on new one, then slowly remove pressure from ratchet and tensioner should put pressure back on belt.