There are many ways you could do this. You could create a variable that is by month and year and use SQL to do the aggregation or you could do this in a data step with by group processing (assuming you had your month & year variables) or you could simply use proc means.
Syntax will be something like...
proc means data=mydata mean; *if you want the default statistics then remove mean here otherwise it will give you the average ratings by month;
format date monyy7.; *This is to get the 12 month categories for each year;
Doing the above avoids having to spend time creating new variables etc and you take advantage of the benefits of using a SAS format for group processing.