Instructions
Read the Forgotten Realms books. Take notes. Decide where you will be beginning your campaign and why the heroes are there. Decide if you want to keep them in that locale or if they are free to traverse all of Faerun at will. If they are allowed to travel freely, have an idea of what is going on in other parts of the realm, so you are prepared for anything that the players throw at you.
Spend some time and create an interesting plot. Start your plot with basic confrontations that players can handle at lower levels, but also impact the larger story arc that spans the entire campaign. While you do not have to have the entire campaign planned out, it is a good idea to have a concept of the overwhelming goal that your players are trying to accomplish.
Consider the players you know and decide who to invite to play in the game. Some players are just not compatible and sometimes a player is just not right for the game you are intending to run. For example, if you have a friend who always, always turns on his party no matter what character he is playing, he might be a bad person to invite to a campaign that requires party unity. If you know a player who only plays evil characters, even when it says good on the character sheet, do not invite her to a goodly game.
Set boundaries. Some gamers, for whatever reason, need to know before committing to a game what the DM expects of them. Let them know from the beginning if you expect them to always be at game (or call beforehand to let you know they can't attend) and what time they need to arrive for games. Some gamers are perpetually late and need to be told that being late delays the game for everyone. When you let people know beforehand what they should expect, few get upset when you enforce the rules.
Assume that nothing is obvious. Explain your personal rules regarding cheating, inter-party conflict and eating at the gaming table. Make the rules clear and enforce them. Your players will thank you for it.