Hmmm...I would say that your choice of materials would depend on what you want to build, where it will be used and for what purpose. You will have to know the properties of the materials you are using. These are like; flexibility, strength, hardness, etc. If you're anything like I was; you will find out the properties by using the material.
A good way to learn design and engineering, is to build models. I built lots of things when I was your age. I used whatever materials I could find; wood, sheet metal, springs, wire, paper, string, glass, plastic, etc. I learned a lot from doing it and when I took engineering classes, I gained a more deeper understanding of materials.
Have fun building!
It is great to hear you love building things, you are already on the path to becoming an engineer. But a couple of things to keep in mind. First, engineers know how to build things, but most engineers are designers. They draw or describe what is to be made and someone else does the actual work of building it. This is because engineers design things that are too big to be made by a single person. Second, engineers use math and science to figure out how to build complex things, so you have to do well in math at school and enjoy learning about science. It takes a lot of school and training to become an engineer, but all good engineers started out just like you ... building things because they enjoy it. What is so much fun about working as an engineer is that we get to design things that are much bigger and more complex than what we could do by ourselves. Working as an engineer is as much fun as what you are doing now. Keep building things.