Probably the best place to start with for the 30 Days of GMing Challenge
What advice would you give a first-time GM?
Don't try to be perfect, especially your first time out. There will be a strong temptation to over prep, to want to know everything before you get to the table. It's never going to happen, and besides, your players will be sending you in more and different directions than you can ever totally prep for.
Roll with the punches. When your players throw you a curve, when you screw up a rule, when something just goes wrong, just keep going! There's an improve technique "Yes, and?" which is a good one to remember at the table. When shit happens, go with it. Don't try to fix it! The bandits overpowered the party? If they're not particularly blood thirsty, maybe they bandage up the PCs and try to recruit them, or hold them for ransom. Suddenly your game goes from dungeon crawl to prison break. If you can master this skill, your players will probably never know things aren't going like you expected!
Go with it. Yes, you want to run Lord of the Rings, or Star Wars, or Game of Thrones, and your players end up more like Monte Python or Knights of the Dinner Table. Epic tales still come out of Monte Python and KoDT!
Above all else, have fun! The whole point of getting together with a group of friends is to enjoy yourself.