When do we use a comma before “for”?

This probably isn’t what you want to hear, but I’d strongly recommend learning comma rules in general, rather than trying to memorize a bunch of conditions for when commas come before certain words. Here’s a good, thorough resource for comma rules, and here’s a more user-friendly one that will probably still suit your needs. That might seem like a lot, but it’s still far less than you’d need to memorize if you try to build up a complicated “when do I use a comma before {word}” system.

All that said, here are a few quick thoughts about your actual question. When “for” is used as a conjunction between two clauses (“for” is the F in FANBOYS) then you need a comma before it. When it’s used as a preposition (e.g., some flowers for my mom) you probably don’t need a comma before it, unless it’s in a list or some other structure that needs commas.