Wow, she did great today.

Wow is what part of speech in the sentence?
I think a preposition but not sure?

Nope. Look up interjections.

My ninth grade English teacher described prepositions as this: anything you can do with a stone in relation to a bolder: on, with, in, upon, beneath, of ...

Thanks!

It's not a preposition. Prepositions begin phrases.

Examples of prepositional phrases:
to the store
in a moment
from my friend
under the bed

Study this site.

http://www.chompchomp.com/terms/interjection.htm

In the sentence "Wow, she did great today," the word "Wow" is not a preposition. It is actually an interjection, which is a part of speech used to express strong emotions or reactions. Interjections are often separated from the rest of the sentence by a comma or an exclamation mark. In this case, "Wow" is expressing surprise or admiration, making it an interjection.