Your review should consist of an introduction, several paragraphs, and a conclusion.The paragraphs should cover following topics: fun factor, control and gameplay, graphics, sound and replayability.
Don't spoil important story elements! Try to introduce the story but don't ruin the game for others by revealing turning points or the ending.
Ideally, you should have completed the game when you write a review. If you haven't, mention how much time you have spend with the game. Additionally, if the game has a multiplayer mode, you should have played it for a fair amount of time and include it in your review.
Try to be objective, but don't force it. Clearly state your opinion.