First of all, welcome to the boards- I see you're relatively new! I hope you get as much enjoyment and just plain usable ideas from this place as I have!
Now, as to tips for your own setting- I recommend starting small. Have a town with a few notes, maybe your first dungeon statted up, a few local npcs (the hermit, the wizard with the tower outside of town, the halfling/gypsy band with their wagons, etc). Most of all,
NEVER THROW AWAY ANYTHING YOU HAVE WRITTEN DOWN ABOUT YOUR CAMPAIGN. You'll wish you had it later, when you're trying to loop the storyline back into it, and you can't remember that ferryman's name or description, or what the potion the pcs got 20 games ago was.
Don't be tempted to give the pcs too much phat loot too soon; if everyone has 1-2 magic items and they're all still 1st level, you may be overdoing it. (Once you have some experience, you may decide that you really like having high levels of magic, but I recommend staying close to book values at first.)
If one of the players (or you) find a rule, spell or combo that seems broken, abusive or not fun, stop and look at it. There are probably reasonable counters built into the game somewhere, you just need to find 'em. A great way is to post about it here on ENWorld.
Don't let rules debates break the flow of the game. Give rules questions about 1-3 minutes at the most; then make a ruling and move on, explaining to the players that you'll look it up after the game and
from then on that's how it will be run. Don't allow take-backs, either- once a die is rolled, be firm that it's too late to change your mind about that action. (You can still, for instance, attack and then decide to move instead of taking a second attack if you're eligible for one, of course.) Don't hesitate to let your players help you out with the rules, though- if one of them is really into bards, and knows lots about bard songs, let them help out when bard stuff comes up. If you don't remember exactly what a bard's inspire courage ability effects but your player does, let him be the 'reminder' for the group.
Most important of all- have fun!
