What are some basic rules when using は.
This is a difficult one because there is some nuance.
は and が just mark the subject of the sentence.
は is used when there's no need to emphasize the word before it
わたしは がくせい です。 I am a student.
So the subject is still "I", but it doesn't matter that you are a student. You could just say がくせい です and the person would understand that you are the student.
When you use が, you are emphasizing the subject more.
わたしが がくせい です。I am a student.
In that example, you are telling the person it is you who is the student. No one else. You.