Start by reading the GMAT and the Share your GMAT Experience forum for a few days. Most beginner questions are answered very well.
Next, take a diagnostic test to assess your strengths and weaknesses. That should give you an excellent starting point.
The 'how long should I prep' question is best answered by measuring your progress. You are ready when your data tells you that you are ready.
Stick to the official material as much as you can in your first few weeks of prep. It will help you learn the basics well. Start with the Official Guide for GMAT review, 11 edition. For additional practice with Critical Reasoning, get the LSAT Superprep
. I would again like to emphasize the importance of tracking your progress.
Finally, participate in the forum. You will learn when you try to explain your answers.
Welcome to GMAT Club.