Your question asked what is the difference between wa and ga. Jury is a teacher, she konws the question and given you the answer. " I think you are the beginner" (quote from nori). it is not an answer to your question. It appears that nori did not understand English very well. She is a Japanese, but she can not under the question where it was written in English, how can she give you the correct answer. Native English speakers understand the concept of the English meanings, Ms. Jura's answer is correctly in response to your question, I go along with her.

If I say: Watashi wa Mark desu. It means that the information I want to highlight is after the "wa". If I say: Watashi ga Mark desu. It means the information I want to highlight is before the "ga". Here are two examples: If someone asks me "What is your name?" and if someone else asks "Who is Mark?" In English, the answer to both is "I am Mark". But in Japanese the answer to "What is your name?" is: Watashi wa Mark desu. Because "Mark" is the information I want to give. For the question: "Who is Mark?" for example when a teacher is asking a group of students, then I will answer: "Watashi ga Mark desu". Because "Mark" is already given, the unknown information is who. So in the answer, I want to highlight "I", therefore I use "ga" to indicate that is the information I want to focus on.

As you just wrote Romaji, I think you are the beginner, so you had better understand Jura's answer first. If you are over that point, you can read this site to understand the difference of "wa" and "ga". http://www.ic.u-tokyo.ac.jp/japanese/b01_04_j.html Jura, I think you knew these already, but it is helpful site, just take a look if you're interested.