Hi, I'm in exactly the same boat as you - 16, just finished GCSEs - although I have been planning my project for a long time.
I'm the producer on my project so I'm in the position of managing the 10 strong cast, figuring out which days we can shoot which scenes - which is a logistical nightmare, given that its the summer and people have got jobs and holidays booked; and keeping my extreamly creative but somewhat tempremental Director from throttling the Director of Photography.
My advice to you would be, once you have the script, hold casting sessions in your local area (put up posters in local drama groups, community centres etc.). Also, its a good idea to have a facebook group everyone interested in the project can join.
Once you have a cast get them to give you all the dates they are away so you can plan around them.
Obviously how much time you need will depend on the length of your script, my project is feature length so we are shooting for the whole summer, but your short might only need a couple of days.
As far as ideas go you just have to be creative, I can recommend an excellent book on screenwriting:
http://www.amazon.co.uk/Raindance-W...=sr_1_1?ie=UTF8&s=books&qid=1244889320&sr=8-1
Or if you just want to mess about with your camera and your imagination and arn't to fussed about writing a script take a look at this compitition from E4:
http://www.e4.com/estings/index.html
Good luck & happy film making,
Smith