Going on a very long flight in two weeks! Nervous?

I'm leaving from Washington D.C on a flight overseas that'll be 17+ hours long, then a layover for 4 - 5 hours, then another 3 hour flight till I reach my final destination. I've flown before a few times but for some reason I'm feeling kinda more nervous this time!! lol. Do you have any advice to calm my nerves? Also, I'll be wearing a nice shirt and jeans but am considering taking sweats with me on the plane to change into - is that a good idea (I have to look good when I arrive though :P)? I'm 14, thanks :)