Just wait on it for now. There are other plants that you can grow that will be easier. I hate how grown-up this is going to make me sound, but you should consider getting a paper route or mowing lawns or some other amateur job and just save for a while. Cooling a terrarium is not easy or cheap and you won't be able to do it just on a whim. If you can take some extra chores from your parents and save just $5 a week, by the end of the year you'll have over $150, which should be enough to buy a quality air conditioner with a built-in thermostat if you shop around during the off season. I don't know how your parents are, but back when I was your age showing responsibility went a long way with mine. If you make an investment your parents may be impressed by your dedication and want to help you get further into the hobby.
Speaking as someone who usually puts things together from scraps, the air conditioner is the really hard part in this type of setup. You're going to have to buy it, one way or the other. Other parts, like a terrarium, can be found cheap or even for free if you do a lot of searching. Ask your parents if you can hunt around on Craigslist for free items; every once in a while people will give away totally good aquariums and other stuff just so long as someone comes and hauls it off. You can at the very least find good tanks that way.
~Joe