Hi Jeannie,

The sun is made of mostly hydrogen, and significant bit of helium and just small amounts of everything else.

There are a number of reasons why people study it. Are few of them are

-The sun is our star and our lives depend on it. It produces the light that heats the earth, ultimately providing it with almost all its energy.

- Solar activity (things like flares and coronal mass ejections) can also affect us, chiefly by affecting our technology - communications, power systems, spacecraft. This is called space weather and you can read more about that here:
http://www.windows.ucar.edu/spaceweather/

-It is the star closest to us and thus the easiest one to study. That makes it important if you want to understand stars, how they are structured and what sorts of things they do.

-The sun is also a very good thing to study if you are interested in the physics of materials at high temperatures - much higher than anything we can create here on earth.

Astronomers, scientists who study the sun stars and other things out in space, have to take a lot of math and science classes.
If you are interested in becoming an astronomer you should try to take as much math as you can in high school and also physics and chemistry.

Cheers and good luck,
Terry