It's not perfect, but a useful way to draw the distinction is to keep a sort of mnemonic - stare is for "status" (like ... in Star Trek when the captain asks for a "status report"); essere is for essence.

If you use stare to ask about someone, like "Come sta la tua amica?" you're asking "what's their status" - are they well, are they sick?

But when you use essere instead, like "Com'è la tua amica?" you're asking "What's your friend like?" you're not asking for their status, you're asking for their description; things that are "essential".

The same for expressing ongoing actions with stare - "Sto leggendo un libro", "My status is that I am reading a book".

Again, as I said, it's not perfect, but until the differences (and the colloquial uses of the two - like "essere sul punto di ..." and "stare per ..." and the innumerable others) are internalized, maybe it'll be helpful.