Both has many meanings depending on the context, and it is not so simple. Basically, "wa" indicates the topic of the sentence while "ga" indicates the subject of the sentence. And when "ga" accompanies "wa" or "mo", "ga" is dropped. For example: Watashi ga onna desu. Watashi wa onna desu. (you cannot say "watashi gawa..") Actually "Watashi ga onna desu." is also grammatically correct, but it can used only when you need accent on "onna".

I could write a book if I try to explain it all. Can you give the particular sentence that you're confused with? Then I might be able to explain why it should be either ga or wa.