Learn Italian: Vedo il fumo: l'incendio sta per arrivare qui! - I see the smoke: the fire is about to arrive here!

You use the verb stare matched to your subject, add the word per, and then use the infinitive (the dictionary form) of the main verb. Here, l'incendio is an "it", so we use the third-person form sta, followed by per arrivare.

Could I say sta arrivando instead of sta per arrivare?

You could, but it changes the meaning slightly. Sta arrivando means the fire "is arriving" right now, as an action currently in progress. Sta per arrivare means it "is about to arrive", an action that is on the verge of happening but has not quite reached you yet.

Why does the Italian sentence use l'incendio for "the fire" instead of il fuoco?

Fuoco is a general word for fire, like a controlled fire in a fireplace. Incendio means a destructive, spreading fire, like a forest fire or a burning building. Since it is producing smoke and moving toward them, incendio is the right word here.

Why is the article l' used before incendio instead of a normal masculine article like il?

Incendio is a masculine singular noun, but because it starts with a vowel, the standard masculine article drops its own vowel and becomes l' so the two words flow together smoothly.