[Imp]

Your FOOs (forward observers) & spotting vehicles are the best units for calling in artillery followed by your HQ unit. It depends on the nation & timeframe but these units have faster call times. If they have line of sight they are also more accurate, they can also adjust fire with less penalty.
These are your go to guys for artillery hence for ease buy them first or last to make finding them easy.

If you have FOOS or dedicated vehicles just use them. If you only have HQ plot fire with them & if you really want to go to the effort see if a unit with LOS can adjust it better.

Planes take a bit of getting used to try not to overfly your troops or plot near them as they may mistake them as the enemy. I would learn the basics before playing with planes.