Just starting keto... would 3 cups spring mix, 1 can of tuna, 1/3 cup cheese and 2 tablespoons of ranch be a good meal?

Did you enter it into your daily log?  Remember you are shooting for fat and protein to keep you full.  The first month it  is more important than a balanced meal (my opinion). 

You will learn that heavy cream and cream cheese are what people eat with a vegetable thrown in :).