저는 : when you try to describe yourself in general. More like a fact...
ex - 저는 사과를 먹었어요. (I ate the apple.)
       저는 키가 작아요. (I am small.)
       저는 그림을 잘 그려요. (I draw well)

제가: when you try to describe what you did or how you are to someone.
ex - 제가 그 사과를 먹었어요. (I ate that apple.)
       제가 좀 작아요. (I'm a bit small.)
       제가 그림을 잘 그려요. (I draw well.)

Hm.... now that I try to explain it's quite difficult... it's just natural for me to know when to use which, but to explain, it's harder. If anyone has a better explanation, please go ahead.

A teacher once explained it to me this way:

You use 가 for normal/usual things and 는 for unusual things/things that are out of the ordinary for the subject to do.

Example:

A Korean mom cooks Korean food (which is perfectly normal and nothing to be surprised about) -> 엄마가 한국 음식을 요리했어요.

A Western mom cooks Korean food (which is unusual, so I want to say "MY mom cooked KOREAN food!") -> 엄마는 한국 음식을 요리했어요.

No guarantees, that's just what I've been taught. :)

This also seems like a good explanation: 
https://wiseinit.com/%EC%9D%80%EB%8A%94topic-marker-vs-%EC%9D%B4%EA%B0%80subject-marker-korean-grammar-vs-grammar-10

I think 이/가 and 은,는/을,를 is probably one of the harder grammar concepts for English speakers to understand because using one or the other has a very subtle difference.  

Look for youtube videos that explain this (BillyGoKorean explains this well).  

And since it looks like you are a beginner, my best suggestion is learn it as best as you can and accept that right now you will not know this 100%.  

With time you will grasp this fully so don't drive yourself crazy trying the perfect this now.

Non-answer, I know.