Earning stars is solely related to how much score you earn. Score is earned for destroying objects, popping pigs, and completing the level with unused birds. Most of the time, earning 3-stars just requires a few well-placed shots and a bit of luck. Use Youtube to find videos to help you easily earn 3-stars on the levels. You only need to get 250 out of the 450 in the game for the achievements. If you get stumped or can't make the perfect shot on a level, just move on. No big deal. See the video below by NextGenWalkthroughs for the the first 60 levels of the game.