A Foul is defined as...

... any illegal contact that gives the offender an advantage or places the offended at a disadvantage.

This means that some contact is not a foul but should ruled as "incidental contact." However, almost all new officials do not call enough fouls. During your first 100-150 games, the best policy is when in doubt, call the foul.

• Never watch the ball -- always watch the DEFENSE in your area.
• Don't get too close to the play -- stay back enough to see the whole picture. (And stand still -- don't be a moving camera.)

• Blow your whistle very loud and long, and raise a fist straight up over your head.
• Spit the whistle out. (Don't hold it. That's why you have a lanyard.)
• Communicate what the foul is so the offending player knows what the call is.
• Tell your partner (and everyone else) what will happen next:
  • "Green 4 is shooting 2 shots."
  • "Red inbound on the side (pointing to the spot)."
• Move to a place between 15 feet and half the court from the table.
• Report to the table if the basket counts if appropriate, then in order the color, number, foul, and what happens next.
• Move to your new position to resume play.

A key point is that the officials exchange ends after every foul. Since no two officials call everything the same, this keeps it fair for both teams. This means as the calling official you might go right back where you were to administer an inbound, or you may have to jog to the far end of the court.