The matter of が & は is still not settled by native grammarians. It's very difficult to explain how to use them appropriately to foreign learners, but good news is that we native could understand what you really mean when you would mistake which is which.

It's very difficult to explain and I think these pages help you a lot. Please take a look. I hope you understand a bit more about wa and ga. は が http://www.italki.com/question/274881 http://japanese.about.com/library/weekly/aa051301a.htm http://www.japanese-language.aiyori.org/article1.html http://www.guidetojapanese.org/learn/grammar/particlesintro http://www.italki.com/discussion/86044 http://nihonshock.com/2010/02/particles-the-difference-between-wa-and-ga/