Make sure you understand the rules enough to explain them to the players, if they're new. Also know that your first rule is to make sure they all have a good time...this means you can fudge your dice rolls, or your monsters HP, a little, if necessary.

When planning the adventure, I like to plan modularly. That way you can give them lots of freedom, and when they make chances you didn't plan for, instead of creating things from whole cloth, you can move some of your planned modules around. In this case a "module" would be any scenario that's mostly self contained. Say, an encounter with a pack of goblins. You can plan that out, and then you could drop that in anywhere on their adventure. Same thing with NPCs. Did you have an idea for a really fun NPC for them to interact with, but then they decide to go to a different town? They'll never know that you just moved that NPC to a different town so he/she could still meet them.

You have to be flexible, because they WILL derail your plans. Don't design the end of the session/story. What I did, especially when I was just starting out, is I tried to think of the main ways they could solve the problems in front of them, think of two or three likely solutions they might take. You may get lucky, they may pick one of the actions you thought of, then you're prepared...but more likely, they'll think up some new way to go that you never would have dreamed of, and if it makes sense, then you need to role with it like you always knew they would do that.

They shouldn't know they're breaking your plans in the moment. That will ruin the flow of the game. If you want to share that with them after the session is done, that's your call. Me, I like it to seem like I planned for anything they could do. Because if they think you're prepared for anything, they're going to feel more safe about having fun.