Hi Yerlan,
Since you and your friend are just beginning, then it would be a good idea for each of you to take a FULL-LENGTH practice CAT Test; you can download 2 for free from
www.mba.com (and they come with some additional practice materials). A FULL CAT takes about 4 hours to complete, so make sure that you've set aside enough time to take it in one sitting. Once you have those scores, you should report back here and we can come up with a study plan. If you'd like to do a little bit of work to familiarize yourself with the content and question types first, then that's fine, but you shouldn't wait too long to take that initial CAT.
Most Test Takers use a variety of resources when preparing for the GMAT (not just 1 book or 1 set of books). While you can begin your studies any way that you like, you'll probably end up having to adjust your plan a bit, based on your strengths and weaknesses. Some concepts will require more time, while others will require less. You will probably end up having to invest in some additional study resources.
4 months is a LOT of study time though, which is good. It's important to be consistent with your studies though - the GMAT is different from the Tests that you took in College.
GMAT assassins aren't born, they're made,
Rich