This is lots of great advice, I've got two things to add.
1) Like said above, slow whistle but when you do blow it, blow it loudly!
2) If its close, sell the call! Often I see new officials have a very close catch/incomplete and they'll blow there whistle and do a weak incomplete sign. Blow the whistle and sell that incomplete sign all the way. Same goes for fumbles that weren't fumbles because the knee was down. Also same for a close goaline call, don't hesitate with your signal, I see guys start to come up with the signal, stop, then put it up. Wait till you are sure then throw your arms up.
Also have fun out there, be relaxed, take your time. Good luck!
