You can't.
How can i improve my chess before next week ?

Here is my tip. At our level (amateur). Opponents will blunder and that kind of blunder are mostly tactics so study tactics it will sharpen you so that you can view your game and find some tactical blows in it. Polgars book is good.Try solving em.GL

IM pfren is correct of course but I've read other advice to do a boatload of tactics and review your opening repertoire which certainly can't hurt. Those giving the advice all stress not studying the night before though, maybe even a 24 hour break before the tournament so you're fresh. Make sure you get a good night's sleep as well. I know there were other folks looking for advice, let me see if I can find some links:
http://www.chess.com/forum/search?keyword=first+tournament

But my opponent is at level National.
then the pressure is on him to beat you. This is your chance to make history...and upset victory. Gl

But my opponent is at level National.
Well that's tough but to do it in a week to be at his level haha.

My opponent is at National Level?? As an Ex altar boy, I can give you last rites, but chess.com has some pills for that 1 week thing. You can get them at the chess store. The're right next to the Flintstones.

So .. I must Solve Tactic and Calm my mind when the tournament start .. ?
and don't forget to get your regular sleep and eat wisely.
Play as slow as you can. REALLY slow.
Before you move, ask yourself "what will he do in response". That really helps me when I play here to avoid obvious blunders, which is much worse than my OTB.

Well, work on your overall fitness. Make sure to do some exercise before you will be sitting for hours.
Get enough sleep. When you are tired, stop, whatever you were doing. Hit the hey and begin again when you are fresh. 30 minutes concentration beats 3 hours of la-di-dah doodling.
Disclaimer: never played a real tournament.
Good luck!

I would recommend buying Laslo Polgars giant Chess book and perhaps get overnight shipping.
Start doing the checkmates tommorrow.
I would start at the beginning and make sure to get in the habit of getting 100 percent accuracy in your solving of the checkmates.
If you feel the need to check your answer at the back of the book then you are not solving accurately enough.
Do not guess at what the answer is instead figure it out.
If you get stuck just take a break and try again later but don`t guess.

I would recommend buying Laslo Polgars giant Chess book and perhaps get overnight shipping.
Start doing the checkmates tommorrow.
I would start at the beginning and make sure to get in the habit of getting 100 percent accuracy in your solving of the checkmates.
If you feel the need to check your answer at the back of the book then you are not solving accurately enough.
Do not guess at what the answer is instead figure it out.
If you get stuck just take a break and try again later but don`t guess.
So in one week he could beat you ?
Next week i have a tournament . !