You can find many search results online and good answers about the general rules. You should first refer to them. Then, if you have any further questions, I recommend you use italki. http://www.japanese-language.aiyori.org/article1.html https://sites.google.com/site/japaneseclasssouthalryohei/class-material/introduction-2/hand-out/difference-between-ha-ga http://www.guidetojapanese.org/blog/2005/02/05/the-difference-between-and/ http://forum.jisho.org/discussion/155/difference-in-usage-between-%E3%81%AFand-%E3%81%8C/p1

I think the best way to differentiate betweenはand が is regard them as THE and A. For example There is A bird あそこに鳥がいます THE bird is crow あの鳥はカラスです