about the particle " ga " , it's like wa to indicate the topic
The difference is:
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.
look at these links to know the difference between the " wa " & " ga "