First, get yourself a rulebook. If you can't find a HS one, get the NCAA one off their website - starting a race isn't all that different between high school and college.
The biggest piece of advice I can give is to make sure you have the correct starting line. Many tracks have about 30 different sets of lines, and you need to make sure you have the runners lined up correctly (especially important when there is more than one way to start a race). If in doubt, check with someone from the home track to make sure everyone is lined up correctly and has the correct break line.
Bring earplugs!!
Practice a smooth, steady voice (calm but loud enough to hear). This isn't basketball - shouting will do more harm than good.
Don't be afraid to give the "stand up" command if something even seems out of the ordinary.
Make sure the gun is loaded with more blanks than you think you'll need (for recalls, false starts, etc.) and make sure they're blanks!!
Be sure to get an "all clear" signal from the finish line & timers before starting a race. If using FAT, have the FAT operator train you in how to use the starting equipment.
Good luck!