It's so difficult one(._.;) When one says "sth ga"or"sb ga",he wants to emphasis the SUBJECT. And when one says"sth ha" or "sb ha",he want to emphasis the following VERB or ADJECTIVE . Oh,and we don't say "Watashi ha 〇〇　ha 〇〇　desu."or"Watashi ga 〇〇　ha 〇〇　desu." Say "Watashi ha 〇〇　ga 〇〇　desu." just like "Watashi ha osushi　ga suki　desu."(私はお寿司が好きです。I like sushi.) ;but the difference is not so important,i think(^o^) Sometimes,even in the same situation,one uses "ga" and another uses "ha". Maybe you can collect my English,please(^-^)☆

Easy. "ga'" is NOT a topic marker. It is an IDENTIFIER. It identifies who or what initiated the action (or equivalent) represented by the verb. "ha" is the topic marker in Japanese.