how to play in a classical chess game ?

Use your time 

i will use my time but for what . what sholud i think in my mind

calculate variations more? when u see a good move, use your time to find a better one?

Just time management in general. In my early OTB events, I found myself playing quickly and ending the game with more than half my time on the clock. My results improved when I started spending more time on each move.

How much time you spend, depends a lot on the time control and how quickly you get to a point in the opening where you are unfamiliar with the move made by your opponent or the position reached.

all other things, relax and go in with a good attitude.   your not going to go to your first tournament and dominate; unless your opponents are pretty low rated.  and your rating will rise as you get use to the slow game- even if you make no other improvements.

also OTB chess is more pleasant, and polite.

Imaak wrote:

"... what sholud i think in my mind ..."

Possibly helpful:

Simple Attacking Plans by Fred Wilson (2012)

https://web.archive.org/web/20140708090402/http://www.chesscafe.com/text/review874.pdf

https://www.newinchess.com/Shop/Images/Pdfs/7192.pdf

Logical Chess: Move by Move by Irving Chernev (1957)

https://web.archive.org/web/20140708104437/http://www.chesscafe.com/text/logichess.pdf

The Most Instructive Games of Chess Ever Played by Irving Chernev (1965)

https://chessbookreviews.wordpress.com/tag/most-instructive-games-of-chess-ever-played/

Winning Chess by Irving Chernev and Fred Reinfeld (1949)

https://web.archive.org/web/20140708093415/http://www.chesscafe.com/text/review919.pdf

Discovering Chess Openings by GM John Emms (2006)

https://web.archive.org/web/20140627114655/http://www.chesscafe.com/text/hansen91.pdf

Openings for Amateurs by Pete Tamburro (2014)

http://kenilworthian.blogspot.com/2014/05/review-of-pete-tamburros-openings-for.html

Chess Endgames for Kids by Karsten Müller (2015)

https://chessbookreviews.wordpress.com/tag/chess-endgames-for-kids/

http://www.gambitbooks.com/pdfs/Chess_Endgames_for_Kids.pdf

A Guide to Chess Improvement by Dan Heisman (2010)

https://web.archive.org/web/20140708105628/http://www.chesscafe.com/text/review781.pdf

More pleasant and polite? Are you sure you aren't talking about the 20th century?

In speed chess you can often play mostly on intuition with very little calculation and you can get away with it.  At slower rates of play that approach will lead to disaster (unless you are a second Mikhail Tal, or your opponents are much weaker than you) as your opponent will have time to find the flaws in your hasty moves.

That is not a very nice thing to say, especially when the OP is asking for advice to avoid that very fate.

A good general rule is to evaluate the position and make plans while your opponent's clock is running and analyze concrete variations while yours is running. Strict adherence to that schedule is of course not possible, but following it as much as you can makes for efficient use of your time.