It is with deep regret that I must inform you that there is NO rule that would tell you what article to pick. There're a great many guidelines out there but upon close examination they are inadequate, in other words they account for a few examples and won't work any further. And how to apply them sometimes is not clear.

I can identify with you as I've been struggling with articles for a long while and I can tell you that reading books/watching movies and paying close (even painstaking) attention to what you're reading/watching goes a long way, compared to learning boring rules. But it is a painstaking process, you have to put your back into it.

I myself am no fan of rules, I can think of a few people who know all kinds of rules and their language still sucks. On the other hand, native speakers, for the most part, know no rules whatsoever and they will never misuse an article.

The main problem with rules, in my opinion, is that they are ever so crippling. There are so many of them and to hold them all in your head is very difficult, let alone apply them when needed. You will spend the better part of your conversation racking your brain trying to come up with the pertinent rule, rather than thinking of what to say next.

You'd do much better to just remember set phrases/expressions/collocations/idioms, I think.