The good thing about GUMSHOE is that it is very easy/forgiving for a GM new to the system.
My advice:
1. Use pre-gen characters.
2. Hit the ground running right away. Have the scenario begin with all the characters together
while something is going on3. If you are planning your own adventure, keep it simple: know the antagonists/baddies goals, know the key floating clues the PCs need to know, and know the characters. Don't overplan.
4. If you are using a designed adventure, just make sure to read the entire thing at least once, and then make yourself a cheat-sheet to use.
5. There are great (and free!) resources on the Pelgrane Press websites. For all my GUMSHOE games, I tend to use sample characters and the GM Cheat Sheet for The Esoterrorists
http://www.pelgranepress.com/?p=3176. MAKE. SURE. EVERYONE. IS. HAVING. FUN.
7. THAT. INCLUDES. YOU.
8. Use the "pulp" rules at first.