First, you need to know their string names. The first string is called the “A” string, which is the highest string. The “D” string is the second one, between the “A” string and the “G” string. Speaking of the “G” string, it's the third string out of the four and finally the “C” string, the final string of the viola.
Second, you HAVE to know the notes. All the notes on the “A” string go with the finger you place on them. the first note is “B”. The second note is “C” and the last note on the string