The short answer:
- を marks the object of a particular sentence:
"Mark eats an apple." Here 'an apple' is the object, so it will be marked with WO.
- が is the subject marker. In the example above, 'Mark' would be the subject of the sentence. It is the one who or what is doing the eating.
- は is the topic marker. This is where most new Japanese students get confused. The main difference between the subject of a sentence and it's topic is that the subject trumps the topic in a particular sentence, so to speak. A topic can span more sentences at once, without ever being repeated again - it's implied. A subject trumps the topic in a way that when they both appear in the same sentence, the topic is the focus of the idea, but the subject is still doing the mentioned action.
The は will in most cases answer the questions "about who/what?" or "regarding who/what?", where the が will in most cases answer the question "who/what?"
This is not nearly enough to tackle the idea of these particles, but in my opinion, it's sufficient to get you going. After many, many mistakes, you'll get a hang of it, trust me :D