I tried to learn to play the violin. 
I have always wanted to try. They are important instruments in many different genres.
How many different stringed instuments are there?
Many! Similar to the violin there is the cello, bass, and viola. The violin is the smallest and highest pitched. 
What would be the easiest to learn, maybe the viola?
The viola is slightly larger than the violin, but I think they would be about the same in terms of ease to learn.