There is a legend about a dialogue between Mozart and a young composer that went something like this:
Young Composer: "Herr Mozart, I am thinking of writing a symphony. How should I get started?"
Mozart: "A symphony is a very complex musical form and you are still young. Perhaps you should start with something simpler."
Young Composer: "But Herr Mozart, you were writing symphonies when you were 8 years old!"
Mozart: "Yes, but I didn’t have to ask how."
This story is almost certainly apocryphal, but that doesn’t mean it is not very much the truth.
You’re probably going to think I’m not being very helpful, and I’m usually very positive and encouraging; but I don’t believe there is anything anyone can tell you here that is going to edify you sufficiently that you’ll know how to write something as complex as a piano concerto upon reading it.
As demonstrated above, If you have to ask how to write something, you’re not ready to write it.
As Mozart may or may not have done with his young friend, I would urge you to try and write simpler things first before trying to tackle a piano concerto. I read elsewhere that you’re only 13 years old, and you have only been composing for a year and a half. Give yourself some time writing smaller things before trying this. You’ll know when you’re ready to move on to bigger things.
However, since nothing I say is likely to stop you if you have your mind set on trying to build Hoover Dam with a box of Lego, as it were, @aMusicComposer has given you some wisdom about not expecting too much from your first effort (with which I concur), as well as some good advice about studying a book on orchestration – and Rimsky-Korsakov’s is a great one for what you seem to be envisioning. As for planning in advance, it appears you already know something of what you want to do as far as basic things like key, metre, tempo, and instrumentation go. Now all you need are some ideas, and no one can teach you how to come up with those.
Good luck to you, and keep us informed of your progress!