Difference between WO & GA in Japanese.
I understand that wo is a topic marker and ga is used for subjective words...
However I'm still trying to grasp a good way of telling the difference. The only way I can currently work it out if i use wo or ga is this.

WO - Something that the verb does e.g sushi wo tabemas ( I eat sushi)
GA - Something that the word does to the verb e.g Sam ga tabemas ( sam eats)

However can i use this method for everything? A good and easily understood answer would be greatly appreciated.

Question

Difference between WO & GA in Japanese.
I understand that wo is a topic marker and ga is used for subjective words...
However I'm still trying to grasp a good way of telling the difference. The only way I can currently work it out if i use wo or ga is this.

WO -> Something that the verb does e.g sushi wo tabemas ( I eat sushi)
GA -> Something that the word does to the verb e.g Sam ga tabemas ( sam eats)

However can i use this method for everything? A good and easily understood answer would be greatly appreciated.

Answer

I explain this to myself the same way. Ha ha.
But this method fails when you want to say, what you like or dislike. Because unlike "to like" and "to hate",  すき and きらい are not verbs but adjectives. So instead of saying "ねこをすきです。" - I like cats. You say "ねこがすきです。" - Cats are liked (by me). 
But that's the only surprise I encountered so far :)

Answer

こんにちは。
When you use "wo", it is used to identify the word as a direct object of the verb.
Ex: パンをたべます。(I eat some bread.)
　にほんごをべんきょうします。(I study Japanese.)
　コーヒーをのみます。( I drink some coffee.)
In English, the verb comes before the object, and in Japanese noun+wo means direct object, so, don't miss "を: wo".
"Noun+が:ga" can be indicate a subject in Japanese.
I think you would like to ask about "Noun+は:wa", as Ryoko-san shows the web-site  Tae-kim would help you a lot.

Answer

I think WO isn't a topic maker, bur WA is said that it's a topic maker. Please take a look at these pages: (わ、も、がhttp://www.guidetojapanese.org/learn/grammar/particlesintro) (を、に、へ、で http://www.guidetojapanese.org/learn/grammar/verbparticles)