When you want to fire at a vehicle treat it like any other unit. One of your units can choose it as a target and must direct all of its shooting at it. Often this means that some of their weapons can't damage it, so assume that the other members of the squad are giving covering fire, bringing forward ammunition for heavy weapons or simply keeping their heads down!
Roll to hit against vehicles in the same way as against troops. Any hits scored must then have to be rolled again to see if they penetrate the vehicle's Armour Value in order to cause any damage as explained later.