If you know anyone who has experience with D&D, ask them for help. Damn near EVERY RPG book that I've ever read, including my oh-so-beloved Pathfinder books, are horrible at aiding newcomers to the game. There are a few key notes to remember...
Character creation is paramount in D&D. I assume that you'll act as the DM during the game, so it's upon you to read and understand the character classes and the roles they play. It's not expected of you to understand everything, but the more you know, the better.
The adventure books will help a lot with the DM aspect of the game. You'll follow them for story info, NPC info and combat info. If you want to throw in extra enemies to fight, the adventure books usually have appropriate monsters and NPC's to use.
Getting a Monster Manual should be your next priority. If you have no monsters, you have no combat.
As a DM, you have a very different role from the players. You must portray the NPC's and monsters, as well as deliver the world itself to them. Don't let the PC's get away with half-assing interactions. When someone wants to sell a bag of jewels, make them encounter the shopkeep and haggle. Encourage them to use the skills and abilities that they have available and find solutions outside of "I stab enemy with sharp thing".
Another thing to remember as a DM, your word is final. As new players, rule disputes are inevitable. Take suggestions and looks things up, but when push comes to shove, what you say is the final word. Don't be afraid to tweak your dice rolls or difficulty checks against the players. The players are the biggest influence on your world, so don't hesitate to turn things in your favor if something goes horribly wrong in your plans.
That being said, don't be a jerk about it. Don't make characters fail simply because you don't want them to. The undeniably best moments in playing D&D come from you and the players role-playing and accomplishing amazing or downright hilarious feats. Cheat when you need things to go a certain way, but don't hinder the players or you might miss out on something hilariously amazing.
EDIT: Can't believe I didn't write about this... Knowing how to go about combat is essential, but there's a lot to it. Learn the bare essentials, attacks and AC, the actions that can be taken in a turn and spells in combat. More complicated combat maneuvers can be looked up as you encounter them. D&D 3 and 3.5 tend to have complicated rules for combat maneuvers, so don't worry about learning them until you need to.