I have a 2002 MTD 18hp 42" rider it broke the drive belt. How do I replace it or should I take it to a dealer.

You could take it to a dealer or try it yourself, it is not all that bad. Start with an OEM belt made for your MTD. Drop the mower deck all the way, it doesn't have to come off but that would give you more room. The main pulley on the engine held the drive belt but the belt running to the mower deck is usually below that drive so it must be removed. I would recommend pulling the spark plug wires from your plugs just to be safe. Disengage the blade and reach up to the spring loaded tensioner pulley between the engine and transmission, pull it back giving more slake to the mower belt. Pry the belt off the pulley while giving the pulley a slight turn. Once the mower deck belt is off you have a open shot at the drive belt. It goes around the engine pulley closest to the engine and then the transmission pulley. It also travels around an engagement pulley that moves when you depress the clutch so here is where a buddy comes in handy. Have your bud depress the clutch so you can see the travel of the engagement pulley. Loop your new belt around the engagement pulley so it get tighter when the clutch is released. Wala, done now just reinstall the deck belt is reverse order. Now is the time to replace the deck belt as long as you have it off you are already half done.

If we're talking about a drive belt and not a pto belt, the double pulley on the crankshaft will need to be dropped down to be able to install a new belt. Impact wrench is the best way to remove the bolt holding it up on the crankshaft.

Here's the best way to do this: http://www.mtdparts.com/webapp/wcs/stores/servlet/KnowledgeCenterArticleView?storeId=10101&catalogId=19502&langId=-1&pageName=en_US/knowledgeCenter/knowledgeCenterArticles/ChangeRiderDriveBelt.html