1) Relax. Every GM, even the good ones, started out as a newbie GM at some point. We all blew it at one point or another and we all learned from our mistakes. Run the game, have fun and the do the best you can, the rest will take care of it self as you learn.
2) Describe things in a way that give the players a framework to understand the situation. Don't say "It's far way", say "It's over 100ft away." Don't say, "It's big and scary", say "It's over 20 feet tall and looks like it has sharp claws and nasty bite." Remember the world only exsists in your head, until you tell the players about it. Give them information they can use to make decisions. Things like distances, size and number of opponents should be quantified whenever it's possible, so the players know what they are facing.
3) If the situation is tricky draw a diagram. It doesn't have to be to scale or even look pretty, but it will help. It's a big difference to see how the river cuts the battle into two halves and how the archers are on the other bank, than to hear it described by the GM.
4) It's not you against the players. As the GM you can beat the players at anytime. It isn't even a fair fight. So, don't fall into that trap of thinking it is you against them. Make it challenging for them, but let things that should work be effective, even if it defeats the monsters. Learn from the mistake and come back better next time.
5) Listen to the players intent. If a player wants to take an action that is not going to work, listen to what they are trying to do, not just what they are asking for. IE, if they say "I hide behind the barrels" and the barrels are only 1 ft tall, don't just rule that they get no cover bonus from the barrels. Point out that the barrels are too small to hide behind, but that they might be able to hunker down behind the crates. Listen to what they want. The player wasn't just saying I want to hide behind the barrels, they are saying they want to hide. A character in that situation would instantly notice obvious hiding places and as the GM, you need to provide that description. Remember the world only exsists in your head until you tell the players about it. If they don't know the scene or don't understand the battle, most of the time it falls on the GM to give a better description.
6) Have fun. Surprisingly this is the biggest problem I find with new GM's. They forget that this is supposed to be fun and we are supposed to be having a good time. Set the mood by being positive and having some humor in the game ocasionally. If something is dragging down the game, make a ruling and fix it. You are the cruise director of the game, make sure your passengers are having fun.
Good luck!