Welcome to EN World, CherryBomb!
I'm a self taught DM, and the first game I ran was an online game using Sunless Citadel. So already we have something in common.

Here are my suggestions:
1. Don't buy any other books right now. Instead, concentrate on learning the core rules.
2. Remember it's ok to fudge dice rolls if you have to. I found Sunless Citadel to be lethal for 1st level characters, and I don't think it's good practice to kill your PCs off right away.
3. In addition to learning the rules, challenge yourself to come up with good descriptions of what your players see, hear and smell. "You enter a room, it appears to be about 15x20 feet" is not nearly as attention getting as "The door creeks open and the first thing your mind registers is the smell of dust. You see before you a small room with small piles of debris in the corners, thickly coated with dust. A few spider webs hang here and there. The only sound breaking the silence is your own breathing."
4. Pay attention to what your players *think* is happening. Often they will come up with ideas that don't have anything to do with the plot you had in mind, but which are far more interesting. You'll want to find a way to tie this first adventure in wtih future ones, so these PC theories are worth jotting down. You never know when you can use their own fears against them!
5. Before you start, discuss with your players how to handle differences of opinions about rules. I'd recommend agreeing to go with the DM's decision in-game, and discussing it as a group after the session. Don't be too eager to create house rules, because as a new DM you won't be able to predict the ripple effect one rule change might have on many other aspects of play.
6. Remember that you aren't out to "beat" the players. This isn't a contest, but rather a story you are all telling together.
7. You don't have to buy miniatures, but I would recommend you buy a large (22"x28" or thereabouts) tablet of graph paper with one inch squares, the kind of tablet one uses with an easel. Any office supply store will have one, and they cost about $20. Then, glue some rounds of plain white paper onto pennies and label them for the PCs and the various monsters they will be encountering. Do this ahead of time, so when combat happens, you can plop the labeled pennies down and move right along. You can draw rooms and whatever else you need on the graph paper, and it will help everyone to visualize what is going on, and what their characters can do at any given time. After you have played for a while, you can all decide whether or not you want to invest in minis.
8. Have fun, and don't sweat the details!