You should try this website: http://www.howtostudykorean.com/unit1/unit-1-lessons-1-8/unit-1-lesson-2/#203 It's the website I use the most when studying Korean. I just started going through the lessons 1 by 1, and I really like the way he explains things. The link I gave you is to Lesson 2, and if you go to the last section, he talks about what you asked! He also has a bunch of other lessons, even getting into some tough grammar in his later lessons! I agree with Gertie though. Don't stress about it! I am still having difficulties with it, but if you use the wrong one, it's usually not a big issue! I hope this helps ^^

The age old question! Well, I'd say do some research and look at it a bit, but I will give you my greatest piece of advice for differentiating between 이/가 & 은/는. Don't stress! Just read and listen. Also, practice writing and speaking and get corrections and feedback. You will get a feel for what is natural over time. Trying to remember rules or an exact formula won't give you a definite answer. There are some key points to each that will help guide you, but overall, just breathe :) ~이/가 - emphasizes noun; often used with adjectives; part of the main structure for various words/verbs (endic.naver.com) ~은/는 - used after noun is already introduced (therefore emphasizing action or other part of sentence); shows contrast between nouns