I want to add a rating-system to my game, similar to how angry birds has it, with 1-3 stars per level. Since I dont really know what this type of rating-system is called I cant search for any material to help me. Does anyone have any example-code or a link to some information about this?
Well, I would expect it to be merely a matter of setting two score (or time, or whatever other metric you want) boundaries for each level. Exceed the highest boundary and you get three stars. Exceed the next boundary (but not the highest one) and you get two stars. Just complete the level, and you get one star. (Alternatively, you could have three boundaries, and include "no stars, but completed" as an option.)
I would expect it to be up to you as a developer to decide where the boundaries are for each level. The actual coding part is trivial, once you've worked out the metrics.