I have to take a 13 hour flights plus two other 3 hour flights and i'm TERRIFIED of planes What can i do to calm myself down or sleep?

I suggest education. Read a pilot's ground school textbook like this http://www.amazon.com/Student-Pilots-Flight-Manual-Certificate/dp/156027719X which may be available at many libraries. With a firm grasp of what makes a plane stay in the air, you can take comfort that as long as there is altitude you are safe.

Also, consider how many aircraft are in the air right this minute. As I write this, it is just over 10,000. See http://www.flightradar24.com/43.98,-100.11/4 Odds are, none of them will crash.

You can get some tablets off the shelf called Kalms; you start taking them the day before and they work for several people we know. You could also go to your doctor and ask if they'd supply some beta blockers as it's just on a seldom use basis.

Fear of flying is one of the most common phobia's. Being out of control seems to play a big part. You don't know or trust the flight crew, and are at the mercy of those in control. The good news is it's very treatable, the bad news is it takes months of what are called "successive approximations" or baby steps. However if you have a good Therapist and stick with it you will reach a point where, though uncomfortable, you can fly with much less fear, and over time beat it all together.