- Core Rule Books
- Gaming Pad
This part can get very expensive. There are some things you can live without initially and some that you can work-around in the short term. Over time though prepare for a hefty investment.
First off, you will need the three core rule books: the Dungeon Master's Guide (DMG), the Player's Handbook (PHB) and the Monster Manual (MM). New, these run $35 each. You may be able to borrow a set from friends or find them used somewhere but you absolutely need all three.
There is also a "book II" of each of these: the Dungeon Master's Guide II (DMG2), the Player's Handbook II (PHB2), and the Monster Manual II (MM2). These are not necessary but they do have some great ideas and expand on the core rules. If you have the means I would suggest getting them right off the bat. If that's a problem, you can easily do without them or pick them up later and incorporate the expanded rules then.
There is tons of additional source material available in the form of books, magazines, modules, etc. Most of it is extremely useful. However, I'd wait on that for now. Get the game running smoothly with a regular group and then look at these things. Even a module may be a bit aggressive for a new group. Get everyone up to speed in the basics first.
Next, you'll need dice. You will need at least one of every type of dice: d4, d6, d8, d10, d12, d20. If you have the funds I would strongly recommend getting 5-10 sets. You will frequently need to roll more than one of a certain type of dice. Each player will need at least one full set as well so if you are supplying dice for them, grab another 5-10 (or more) sets for them. One set of dice, including two d10s (one marked in 1's, the other marked in 10's) to be used as percentile dice (d100) run around $5-$10.
Probably the largest investment will be in miniatures. These run from $0.80 to $200.00 plus for each depending on the quality, size and complexity. It's nice to have a distinctive miniature for each character that represents that character's race and class. With a dozen or so miniatures you can probably offer enough choices to make your players happy. For monsters, it is helpful to have a miniature that shows what the monster looks like and it's relative size. However, it is very common for DMs to use dice to represent monsters. Get six each of various colors of d6 dice and you should be fine. Then start adding one of each monster type as you can.
Some miniatures are made of metal. Others are made of plastic. Some are painted. Others are not. Complicated ones are made up of multiple pieces that must be assembled and glued. And the quality varies greatly. The links section on this site lists a number of miniatures sources if you are having trouble finding quality pieces. For a good quality adventurer piece, expect to pat $2-$5 or more.
Lastly, you will need a gaming pad. These are flexible pads that roll up and have either a square or hex grid printed on them. Make sure that you get one that is designed for erasable markers. These are fairly inexpensive and incredibly useful.
Beyond that you will need loads of paper, pens, pencils and erasers. Each person will most likely need a calculator. Every player will need their own copy of the PHB but that is something you should expect them to purchase. I find that as DM a notebook computer is invaluable! Many players will find that one would benefit them as well. Therefore, a wireless router for internet access for everyone would be handy.