Get the three core books (Player's Handbook, Dungeon Master Guide, and Monster Manual) and a set of dice. Also, try to get some of those little sticky bookmark tabs, I use those all the time with my MM to mark pages of monsters I will be using that session.
Run a few games. If you bought The Sunless Citidel, run that. Try making some adventures youself, they don't have to be anywhere near as detailed as a published adventure. Some notes and maybe some maps will be all you need. Get feedback from you players on what they liked and disliked.
After you have been running games for a few months, if you like DMing and want to keep going with it, I think you should buy a copy of
Robin's Laws of Good Gamemastering. I didn't read the whole thread, so I don't know if anyone else has suggested it yet, but this is one of the best books you will ever read as far as running games. Be advised that the book was intended for people who have been DMing for a little while, and want to improve. It's really not for brand new DMs. Just keep the title in mind for later.
BTW, if money is short, check your library for copies of the core books. It's kind of a long shot, but one of the two libraries in my town has all three books. It's worth a look, you can't say no to free D&D books!
Short Version:
1. Buy core books
2. Run games
3.
Buy this book!
The most important thing to remember out of all this: HAVE FUN!!!