Read the adventure so you’re familiar with all the important bits
Don’t be afraid to go off script and improvise. It’s ok for the players to do something not in the book: just wing it and see where the session takes you!
Bookmark some random tables (I love the Random Settlements in the DMG) and level-appropriate enemies to help you improvise better.
If you followed the above advice and now your players are off doing some little self-selected side quest in some random town you rolled up on the spot, use the Three Clue Rule at the beginning of the next session to get them back on track.
DMing is a skill. It takes practice. You’re probably going to suck for the first few sessions, if not the first campaign. So relax, don’t be too hard on yourself, and have fun. Eventually, you’ll figure out your own unique style. If you end a session near tears because you’ve concluded you’re the worst DM of all time and are considering quitting forever, only to be interrupted with, “When can we play again?” it means you did a good job.