Por + infinitive; Para + infinitive.

[AngelicaDeAlquezar]

As far as I can tell, it's only regional use. In Spain it's more common to say "estar para" and in Mexico (and probably most of Latin America too) we rather say "estar por", when we mean we are about to do something.

- El tren está por/para salir. -> The train is about to leave.
- La cena está por/para cocinarse.* -> The dinner is about to be cooked.
*Notice that a person can cook, but the thing can only be cooked.
- Estamos por/para terminar. -> We are about to finish.
- Estoy por/para salir. -> I'm about to go out.


When something or someone isn't quite ready yet, we tend to use "todavía"/"aún":
- El tren todavía no sale. -> The train is yet to leave.
- Todavía no está la cena. -> Dinner is not cooked yet.
- Aún no terminamos. -> We still haven't finished.
- Todavía no estoy lista para salir. -> I'm not ready to go out yet.


"Estar para", at least in Mexico, can be used to express that we're ready or in disposition to do something.
- ¿Qué tal estás para una tarde de dominó? -> How would you like a domino afternoon?
- No está como para comer afuera. Estamos en cuarentena. -> It's not approppriate to eat out. We're quarantined.
- Ana quería contarme de su pleito con Juan, pero para chismes estoy yo. -> Ana wanted to tell me about her fight with Juan, but I'm not in the mood for gossip.