Usage of the 이/가 and 은/는 particles is a rather big subject which will take some time to master, so you should give it a good amount of time and not be too impatient.
은/는 (the topic marker) sets the topic of the sentence. It selects a subject which is already relatively familair and says more about it treating it as the central theme. Since the subject gains the focus, it has the side effect of contrasting that subject against other comparable things, which is another important function of the particle.
- 저는 요르단 출신 아니사라고 해요 - Introducing yourself, a typical 은/는 usage.
- 나는 지금 갈 테니까 넌 나중에 와 - I do this, and you do that - contrasting two things.
이/가 (the subject marker) is different in that it doesn't hightlight the noun it marks so much but rather focuses on the fact itself. Some fact is mentioned because it is necessary, but the thrust of the idea is not necessarily the subject noun itself.
- 네가 그렇게 말해서 난 그런 줄 알았더 - The main point is about 나. "네가 ..." provides necessary information.
- 어젯밤에 화재가 발생했다 - Focuses on the news itself rather than the fire's cause or details.
- 어젯밤의 화재는 방화에 의한 것이었다 - Now takes the fire as the topic and goes deeper into it.
But there are also many cases where it doesn matter much which of the two is used, as well as those where not including either sounds better.
- 선생님 저 이제 갈게요. Neither the contrast effect(은/는) nor the focus on the fact(이/가) is desired.
- 나 이 땅에 태어나 이제껏 살아오는 가운데 ... Omitting details like 은/는, 이/가 might make it sound better sometimes, so just 나.
That was a very short introduction to 은/는 and 이/가. Please feel free to ask any specific questions you might have.