Hi Deanna,
The main way we can tell what the Sun is made of is by studying its spectra - the light it produces.

You can spread out light into its different colors with a prism or something called a diffraction grating.
This is what is happening in a rainbow (in that case water droplets are acting like a prism).

If you spread out light from the sun well enough you can see that there are lines in the spectra. (See http://www.noao.edu/image_gallery/images/d5/suna.jpg for an example). Those lines and other bright lines that we see in other parts of the spectrum can tell us what elements are present in the Sun.

Here are instructions for building your own spectroscope:
http://solar-center.stanf...activities/cots.html

We can also study other stars this way. It is one of the important way we classify stars. We find that many stars do have a different mix of elements than the Sun does, although all "middle aged" stars are mostly hydrogen*, like the Sun. We can use the study of what elements are in other stars to understand when they were formed, how old they are and information about their life histories!

Terry

*hydrogen is the lightest element. The Sun also has some helium, and very small amounts of other heavier elements.