To understand this, you've gotta be able to understand the difference between the particle -는 (neun) and the particle -가 (ga)
-는 (neun) places the emphasis on the verb- what is being done.
-가 (ga) places the emphasis on the pronoun- who is doing it.
저는 (Choneun) is the polite version of 나는, which means "I" (with emphasis on the verb)
For example:
-어제 뭐했어?
Ojae mwo haesseo?
What did you do yesterday?
If the person is around the same age as you, or younger you can say:
나는 서울로 갔어
Naneun Seoul-lo gasseo.
I went to Seoul. (The emphasis of this sentence is on the fact that you went to Seoul)
If the person is older than you, or more senior, you can say:
저는 서울로 갔어요
Choneun Seoul-lo gasseoyo
This still means "I went to Seoul" but it's more polite.
Like I said before, -가 places the emphasis on who is doing the verb.
재가 (chega) is the polite version of 내가 (naega)
For example:
여기서 호주사람 있지?
Yeogi seo hoju saram it ji?
There's an Aussie here, right?
You can answer:
응, 내가 호주사람이야. (informal)
Eung, naega hoju saram i ya.
Yeah, it's me that's Australian.
or:
네, 재가 호주사람 입니다. (Polite)
Neh, Chega hoju saram imnida.
Hope this helps!