How do you measure a first down?

Answer 1

A team is awarded a first down when they advance the ball ten yards beyond the point where the previous first down was awarded (or where the team initially gained possession).

A team gets four opportunities to advance the ball ten yards, either by carrying (i.e., "rushing") or by passing the ball. Each opportunity is called a "down." If the team fails to gain the required ten yards using all four downs, then the ball is turned over to the other team.

Answer 2

10 yards is a first down.