Best wishes, there is no need to be nervous about it!! You cannot lose anything, even if you lose.
You are nervous because you don't know what to expect in such a big tournament... but realize, all tournaments are the same, there's no difference between a 2 dollar tourney and a 250 dollar tourney, of course the average skill level of the players might be SLIGHTLY higher in a 250, but its not gonna be a totally different game. It's still just poker, it's the same game and that's what you have to constantly keep in mind.
I think after you play in the tournament for a short while, you will realize these fact and that will allow you to calm down and play your best game... I rarely get the opportunity to take big shots like this, and when I do, it never seems to work out well... but that's okay! Embrace it, use it as a learning experience if nothing else... and good luck to you!!