Amy...welcome, welcome, welcome. I have been leader of my group since January 2002, it has been quite the ride.
One thing that I would say to any new leader is take the time to truly learn the way things work in your building, why things are done certain ways, are there policies about certain things...BE WILLING TO LEARN!
Now for some of your other questions...our calendar is often set around district dates, example we have fun nights on conference nights so they are set. Open house night is along with our bookfair, things like that...set your major dates and then fill in. We don;t usually set the meeting dates for the year because something always comes up!
For organization I use a big three ring binder with dividers with each event listed on it, a contract section, a flyer section, volunteer lists section, whats happening section, fundraiser secton, and minutes and agenda section. Keep copies of EVERYTHING, even if you think they are useless keep a copy of it, make a report for every event...on that lists expenses, volunteer count, games, activities, snack (how much bought how much left) any notes to complaints or changes.You will be glad that you keep detail reports!
As far as ideas, this site has been the best thing for me...it has shown me that I am not alone, the sharing and caring is so great that you can learn so much heck I am still learning.
It takes alot of time and you may feel like it comsumes your life but at other times it rewards you.
One last thing is ALWAYS TELL PEOPLE THANK YOU...LET THEM KNOW THAT HOUR MATTER...YOU WOULD BE NOTHING WITHOUT THOSE VOLUNTEERS!!! Good luck and remember when you stop learning you stop growing!!!