There must be a hundred of these threads.
Simple Tips:
1) Start small - don't decide to create a massive home brew campaign world and not start playing until you finish it. The idea is play, either choose a prefab world or make up a small piece of a home brew and build it as you go.
2) Start small - likewise, don't stat up the Wizard/Lich/Vampire BBEG the party will take on when they reach level 19 now. He should just be a name that people who work for him know. None of his 1st level underlings should have met him.
3) Don't over plan - players never go where you want them to go. This is not a bad thing. Just don't spend 90 hours creating the perfect dungeon crawl and get upset when they decide to buy horse and ride to the next town because they've heard the wine is better there.
4) Don't under plan - This is actually harder to deal with. But don't be afraid to tell the players that they've gone somewhere you didn't expect and that you need a few minutes to think about it.
5) Write down every good idea your PLAYERS give you - They sit there at the table with you trying to figure out what's going on and while doing so they come up with ideas you never considered and which are better than you considered. Steal these ideas and spring them on the party in a later adventure.
6) Keep it Simple - don't create elaborate plots with 100 red herrings unless you like to annoy your players.
7) Keep things moving - Every game put out by Steve Jackson Games has the 50/50 rule in it. Every GM should know it. If you have no idea how something should work that the player attempt, roll a die behind the screen and if it is in the high 50% he succeeds otherwise he fails. This assumes there isn't a rule for the action - does the princess blush at his galant advance? You could use Diplomacy versus Sense Motive but that doesn't tell you whether or not she is flattered at the attempt, just whether she recognizes it as flattery.
