I understand how you get confused between WA and GA.

For instance,

if you want to say that I went to a supermarket in Japanese.

you can say

: Watashi Wa Supermarket Ni Ikimashita. (This is just you went to a supermarket)

: Watashi Ga Supermarket Ni Ikimashita. (This is you went to a supermarket as a representative of your family or friends)

Please feel free to contact me, If you need any help.

Kare-WA ii hito desu - (what concerns him) He is a good man.

Kare-GA ii hito desu - He (wspecially he, the guy we are speaking about) is a good man.

More example:

Ore-GA ano-hito-wo atta no-WA kinoo datta - What concerns me (if that)  I met  him, (it) was  yesterday.

You may also meet such structure as "Nani-nani-ga nani-nani-wo suru ... WA  and then long sentence"

It may be interpreted as - What conserns "Nani-nani-ga nani-nani-wo suru" its a "and then long sentence".

In some way you may easily  say about something, then put WA after it and then think about and continue of what you would like to do with that WA.

Sorry for not having JP-layout on my computer.

Hello. I am a Japanese ,and I actually have never thought of that.

We Japanese don't learn about Japanese grammar , so I don't know how it differ in grammar, but we actually feel the differences.

I feel the differences is about enphasisstress.

a) わたしはスーパーマーケットに行きます。

b)わたしがスーパーマーケットに行きます。

I feel each sentence emphasizes bold word.

So usually when we want to omit a, it's gonna be 「スーパーマーケットに行きます。」,and when we want to omit b, it's gonna be 「わたしが行きます。」.

That is one of the hardest to understand things in Japanese.

IMO there are two problems:

1. The main difference (as for sure you already know) is that は is the 'topic marker' and が is the 'subject marker'. You must try to understand what does that 'topic' thing means for Japanese people. Basically you can think about it like a cloud floating over the conversation. If the other person, in a sentence, does not specify what he/she is talking about, that is because he/she is talking about the current 'topic'.

https://en.wikipedia.org/wiki/Topic-prominent_language

2. You can find sentences which seems to broke the point 1, but that is because both は and が have other uses, like expressing contrast with は or using always が in subordinate clauses.

My advice: Maybe is a bit soon for you to try to understand all those things now. Just try to understand the 'topic' thing, but do not worry too much about it. With time, those 'other uses' will appear in your books, and with time you will use は and が as topic/subject markers correctly the most of the times.

here's another link that I find useful:

http://lingwiki.com/index.php?title=Japanese_Particles

there is a section (though not very long, but maybe sufficient) to contrast the WA and GA.