"Thanking you for your time and assistance"
It sounds strange. Maybe this is a common closing salutation, though? I would not use it. I would take the advice from Ly Thu Huong.
However, maybe I am wrong!
Anyway, I think it is better to write thank instead of thanking.
At the beginning salutation, capitalize every word:Dear Sir Or Madam:
I have recently learned about...
This sentence is correct: I find it very useful.
However, I think you should add one more sentence to the end of that paragraph before you end the paragraph.
Use OF here:
and a little bit of Spanish.
:-)