How would you fix up an old treehouse? (read description first)?

I found an old treehouse with a swing. The swing is perfectly fine, but it's the treehouse I'm worried about. I had found it when I first moved to the town, but I thought it belonged to somebody. Today, I learned it didn't. So, I really wan't to fix it up. But, I don't have any tools or wood or anything. I just want to make it sturdy because it's REALLY old and it creaks when you swing on it. I really wan't to do this. It sounds like so much fun. It could be my secret hide-out! I'm nine years old. My parents know about the treehouse, but I already asked them to help me out and they said, "Fix it yourself." So, I just want to make it a club meeting house that can fit about three people in it. It's enough for three people, but I'm not sure about the weight. I mean, it's perfectly fine. I'm just paranoid about the creaking because it's super old.