Every 1st down, you must then run 10 yards. For each time the ball is called out of play, you mark how many yards are gained or lost, and it moves to the next down.

So as follows.

1st and 10 <--You always start on 1st down with 10 yards. Say the ball is run for 3 yards.

2nd and 7 <--you are now on your second down with 7 yards to the next first down.

If you get to a 4th Down you have one more chance to meet your goal of X yards. You can punt it, try for a field goal, or try to run it as a regular play.

If you punt it, the ball will go further, if you make a field goal, you recieve three points.

BUT if you try for a 1st down, you you must make the first down of X yards or the ball will be turned over to the opposing team.