Q: Isn't being a DM a lot of hard work?
A: Yes, but it has its rewards!
Q: What do I need to DM?
A: Theoretically you need the players guide, the dungeon masters guide and the Monstrous Compendium.
Q: What do I _really_ need?
A: As well as the above you will probably need a game world to set the adventure in, this will give you background information about the creatures / habits / society of the world. Game world information is usually found in a boxed set, the most common being Forgotten Realms.
Second, you will need an adventure. These are usually designed for a specific game world, and a range of player levels. This information is generally found on the cover of the adventure (termed as `a module').
Only get the other rule books when you think you want them. Don't think that because you don't use all the rules you're not playing AD&D. Not all of the rules may make sense to you, or they may make the game slow down. Rules aren't important, the game is!
Dice, pencil and paper are all important. At least two of each type of dice would be useful, and loads of d6.
Most importantly: imagination and time.
Q: Isn't that going to cost a lot of money?
A: Yes.. it will. There are ways around this, first if you are going to play with the same group of people you could all chip in and buy the books (This is not common practice however, and can sometimes cause arguments). If you are at school/university and/or part of an RPG club you may find that the club has funding and you can buy books with that.
Remember though, that apart from the players book (and possibly the DMG) they are all reference books, which you can get by without. Also you do not need to purchase everything in one big go, most people accumulate stuff through time.
Q: What do I need to prepare?
A: A world - you should have a world designed, with inhabitants, societies, politics, attitudes, ecosystems, climate, history, etc. Now this sounds a lot, but you only need to go into heavy detail if it will effect the players. This is the advantage in playing in a TSR world. You just need to make the players feel that it all exists.
An adventure - what are the players actually going to do? The adventure should have some sort of motive for the players to actually do it, and usually has some sort of reward at the end. Again TSR has pre-prepared adventures.
Q: How many players should I start with?
A: As few as possible! It's not easy controlling 8+ people, in fact it's a mess (I've played in one game that had about 12 players and that was just daft).
Usually between 1-5 players produces a good clean game, where all of the players can be heard. After you become more experienced it will generally be between 4-6 players, with more than that the players have a lower chance of getting heard (and combats take forever..).
Q: Are there any DO's and DO NOT's
A: Yup,..
DO be neutral and fair. You can not favour any player over another, that just goes against the whole spirit of the game, breaks friendships, etc.
DON'T play a game of DM verses players. The DM can always win.
DO listen to players criticism. Find out what they like and don't like. They have ideas on how to improve the game too.