Here is a simple way to remember it that works most of the time (and if you're correct over 50% of the time, you're ahead of random chance, which ain't bad):
Just remember that "g" comes before "h" in the alphabet. "G" comes before...."H" comes after.
"Ga" emphasizes what comes before it. "Ha" emphasizes what comes after it. This is a communications task that we usually do in English by stressing words:
JOHN is a teacher.
John is a TEACHER.
Same exact words and construction, yet communicating two different things.
Who is a teacher?....JOHN is a teacher.
What does John do?....John is a TEACHER.
Now imagine if you swapped the answers to those two questions and how odd and unnatural they would sound. Japanese native speakers probably find は/が equally as jarring and for the same reason.
I've never been smart enough to understand all that "topic marker" and "subject marker" stuff.
Please avoid the universal habit of translating は into "As for xx," every time you see it. It is annoying, unnatural, and serves no purpose. So long as you're aware of what communicative function is being done, go ahead and translate it into natural English. Or, better yet, don't translate at all.