Your parents realize that you are turning into a responsible young adult and want you to prove it. You are going to be in high school next year and they think your room should reflect that. They are giving you the opportunity to redo your room from scratch. They are going to completely gut your room and let you redesign it the way you want.
What's the catch?
Your parents don't have the money right now. They need to take out a loan for this. That means that there will be interest and they will have to pay monthly payments. You need to keep that in mind and be reasonable with what you buy. Your parents believe in you but it is their money so you need to get their approval before the construction goes underway.
How do you get your parents to approve your room?
You are going to write a letter to them trying to convince them that you made good decisions when designing your room. You need to tell them what you are going to buy, what it will look like, how much it will cost, how much interest there will be on the loan, and how much they will have to pay every month. Keep in mind that they may need some convincing but if you do a good job, they will appove your plans and give it a go!