

Critical reasoning questions test your ability to analyze logical arguments. The arguments cover a range of topics and situations which average GMAT-takers would be expected to be able to understand, even if they are not very familiar with the subject area.
You do not need to know formal logic. You do need to be able to evaluate a piece of reasoning.
Try the questions in the minitests. Read the explanations to the ones you get wrong.
Before taking the test you should do all the critical reasoning questions in The Official Guide for GMAT Review, 11th
Edition
. The explanations written by the test-makers are very helpful.
If you need more advice about how to approach critical reasoning you could try Master the GMAT, 2007 w/CD-ROM (Master the Gmat) or Cracking the GMAT with DVD, 2007 Edition (Graduate Test Prep)
Analyze the information on which each question is based, and then choose the most appropriate of the answer choices.