I have to tell you writing grant proposals is a daunting (look that up) for a twelve year old. But, you can do it.
I will try to get some professional help on your question, so check back tonite.
In the meantime, be certain of the following:
Your cost estimate.
Other funding you have tried to find, and failed.
What is to be gained from this experience.
This is probably the clearest set of directions I've seen, but even though it's good, there are still a few things missing.
Here are a couple of things I have done, and with decent success, I might add.
1. Before you try to tell what the problem is or why you think you need the money or what your solution is, you must describe the current situation in detail.
In other words, you must tell "what is" before you can even begin on "what should be" -- got it?
2. It also helps if you tell a little story. What I did with one grant app was to state the situation of the school (demographics, finances, graduation rate, etc.), then told a little story about a typical (hypothetical) student, where he was headed without any guidance, where he actually headed once guidance and relevant coursework were put in place, and how all that illustrated the power of what our school would do with the grant money.
Once you do those things, then follow the list that Bobpursley gave you above.
Please re-post if you have questions.