Game reviews are found on the game pages and in the Reviews Module which is shown by default on the Front Page. When a review is accepted by GeekMod the user who submitted the review is rewarded with 1-5 GeekGold.
To submit a review, simply go to the forums of the game it is for, select Post, and in the "Select a Forum" pulldown menu, select Review instead of the General forum.
If you would like to write a review, please pay attention to the quality of your writing. Avoiding grammar and spelling mistakes will make your review much easier for readers to understand and enjoy. Good writing may help also you earn more thumbs and GeekGold. If you are unsure of your writing skills, you might find Common English Mistakes to be helpful.
You can see all the reviews a particular user has written by going to their User Profile, clicking the User Contributions tab, then clicking Reviews.
If you want to write a comparative review (e.g. Dare to Compare: Three Strategic Games on the American Civil War) that substantially covers several games equally, so that it would be of equal interest to readers about all the games discussed, you can submit it as a review to the primary or most popular game covered by the review, then make posts in the general forums of the other games covered in the review and ask an admin to move those link posts to the review forums (as discussed here). This avoids you submitting the same review in separate forums, where separate disjointed discussions would ensue instead 1 unified conversation, and it avoids the impression that you're trying to sneakily earn geekgold multiple times for the same review. See this thread for further discussion.