Just so you know, I'm a dreamer. I'm always dreaming of things that are like castles in the sky. I'm also an employee at a high-tech company, where we get paid to turn dreams into products. That is a tough process, because you always have to cut away a lot of the shiny stuff from your dream, and only leave the little kernel that can actually be made into a working product in a reasonable amount of time. That's the stage where you are now.
Thought-controlled wheelchairs do not currently exist, so that is a fantastic idea, one that would make you rich and help millions of people around the world. However, they don't exist because many of the things you need to make one haven't been invented yet, or are very complicated and expensive, or only exist in big, hairy and impractical prototypes at big-time research institutions. Translation: an 11 year old with a budget of $100 and a few months is not going to beat them to the punch.
Nevertheless, you only need to do what the big guys do: divide and conquer. The people who are working on thought-controlled vehicles and machines start out with the simplest possible devices. For example, they get an alpha wave headset that can be attached to a computer, and write the simplest possible computer program that will move a cursor or something on the computer screen with the brain waves that are being read in from the headset.
Think about it: Once you can write a program that can move a cursor, you are only a few steps away from raising and lowering a mechanical arm. Once you can raise and lower a mechanical arm, you are only a few steps away from making it also swivel on a base. Once you can do that, etc. etc. See what I mean? If you do the simplest possible thing now and each year add something new and enter it in a science fair, by the time you are a senior in high school you may well have a chair that can go forwards and backwards and turn left or right using brain waves. You may also have a mechanical arm that can grab something and move it around in some simple way. If you can get there by the end of high school, the sky is the limit after that!
Divide and conquer means one step at a time. You might as well start now. Do you know what you could use to read brain waves into a computer? Do you know any programming languages? Make sure you know how to do an engineering project(http://www.sciencebuddies.org/science-f ... ring.shtml