When you want to fire at a chariot, treat it like any other unit. One of your units can choose it as a target and must direct all its shooting at it. Roll to hit and to wound against the chariot in the same way as against troops. Any hits scored are targeted towards the chariot as a whole, unless any characters are riding in the chariot (see Characters in Chariots). Some chariots are large targets and you therefore get a +1 to hit bonus when shooting at them. If the chariot is a large target then this is clearly indicated in the army list entry.