Study some magazine articles that review computer games.

There are plenty of online reviews also to look at.  Try http://www.reviewcentre.com/flist16-f1.html

You will notice that most reviews are written in a similar style.  There are usually some ratings (either out of 10 or with stars) for things like graphics, gameplay, sounds, value for money etc.  There is usually an indication of which genre the game belongs to. There will be some general information about the storyline and characters and the good and bad points for the game are highlighted.

 

Choose a game that you have played.

Write a magazine article reviewing the game.  Include information about the setting, the characters, what the player has to do, the goal or objective of the game, the storyline, reasons for it being good and bad and a recommendation to others about it.