I'm glad you want to tell God your thoughts—and I hope you will always want to, even when you grow up. I'm afraid many adults forget God when they grow up, but I hope you won't.
But you don't have to write a letter to God to ask Him your question—and the reason is because you can talk to Him and tell Him what you're thinking. This is what we call prayer. Prayer is simply talking with God and sharing our thoughts and worries with Him—and when we sincerely do that, He has promised to hear us. The Bible says, "This is the confidence we have in approaching God: that if we ask anything according to his will, he hears us" (1 John 5:14).
You see, God isn't just another person—even a great person—who lives somewhere on this earth. God is far greater than we are, and even if we can't see Him, He inhabits the whole universe. In fact, He made you and me, and everything we see around us. I know that is hard to understand—but that's how great God is!
But let me tell you something else about God that is also very important: He loves you. That is why you can pray to Him and know that He will listen. He also has written a "letter" to us: the book we call the Bible. Ask your parents to get a Bible story book for you (perhaps at your local Christian bookstore), and as you read it you'll discover just how much God loves you, and how you can love Him in return.