you get 4 downs to get 10 yards.
Usually, if you don't convert for a first down after 3rd down, you punt on 4th down. To make it less confusing, substitute the word Down with the word Try. You get 4 tries to get 10 yards. Now, if you get tackled behind the line of scrimage, that will add yards to the 10 you already have to get. The line of scrimage is where the ball is.
So, if you run a pass play but your quarterback gets sacked 5 yards behind the line of scrimage, you now need to get 15 yards for a first down. so it will now be 2nd and 15. on your 2nd try, say you do another pass and it is completed 12 yards down the feild. On 3rd down, you are now 3rd and 3 because you have 3 yards left to get in order to convert for a 1st down. On your 3rd try, say you attempt a run, and get tackled a yard behind the line of scrimage, you lost a yard so now it is 4th and 4. You would then punt the ball away because it is too risky to try to convert. If you were to go for it and fail, the other team gets the ball where it lies.
One of the most confusing concepts of American-style football is the down-and-distance system. Every time a team takes possession of the ball, it is given a set of four downs, or attempts, to move the ball 10 yards. If the team can move the ball 10 yards or more within four downs, the team gets another set of four downs to go another 10 yards, and so on. For instance, if a team advances 3 yards on first down, the next play is second down with 7 yards to go (second and 7); if the team then advances 5 yards on second down, the next play is third and 2; if the team then advances 2 or more yards on third down, the next play is back to first and 10, with a whole new set of four downs during which to advance the ball.
After each play, the officials determine how many yards a team has advanced or lost (a team can lose yards if the ball holder is tackled behind the line of scrimmage -- this line is discussed in a moment). The officials then place the ball at the point where the team has ended up. This point determines the line of scrimmage, which is an imaginary line that runs across the field and is the starting point for the offensive team on each play. On the sideline, a team of officials handles a 10-yard-long chain, which designates that 10-yard mark a team must reach to get a first down. On close plays, this chain is sometimes brought onto the field to measure the distance from the ball to the 10-yard mark. The nose of the ball must reach the bar connected to the end of the chain for a team to be awarded a first down.
If a team fails to gain 10 yards after three downs, it may choose to punt the ball to the other team. If it doesn't punt and chooses to use its fourth down, or "go for it," it must reach the 10-yard mark or it surrenders the ball. A team often chooses to punt the ball in order to back the opposing team up so that it has to cover a greater distance to score. The team receiving the punt can return it, meaning it can catch and run it back down the field. The kicking team is hoping to kick the ball down the field and tackle the receiving team's kick returner before he comes back down the field.
If your up for a little more reading, take a look at this... First Downs