Help me how to begin the study !

I would suggest you give the diagnostic test (the first test you will find on OG-11) and find out what are your weak areas. And then focus on that. If you want to start as a complete beginner then start with Manhattan SC book for verbal and Manhattan number properties for quantitative. At the same time review all those basics covered on both OG. Meanwhile you can do few questions everyday to find out your improvement.

I would do all of the Manhattan guides first because those are designed for building fundamentals.

Then I would take the diagnostic test and work through the guides.

Finally, you can simply download the GMATPrep software from MBA.com

I would suggest, if you want to save money, to go to amazon.com and buy a bunch of outdated (like 2005 and later) books with CD practice tests that are like $5 a pop, just make sure they include the CD. I just recently scored a 700 (45Q 41V) my first time out (I'm going to be a 20 yr old Junior in Undergrad Fall 2008) off simply studying from prep books with CD practice tests. I would suggest getting at least three different test prep providers tho, because each one has its strengths and weaknesses. I prefer Kaplan, because their tests are always harder than the real thing (I am getting like 590-600 results on them, whereas I am getting like 750-770 on Princeton Review), but some of their questions I do believe do not have the correct answer. Arco is also a good name. I also found the CAT software on the actual GMAC practice tests you can get mailed to you once you pay your $250 and schedule a test very helpful, and they can almost pinpoint your score.
I find that taking the GMAT is more an endurance test than anything else, and once you find out the types of questions and a methodology to answer them, it is more practice makes perfect than anything else. my practice scores go up practically every time I take a test (like once or twice a week), with little to no outside studying. I do read these forums and try to help answer questions in my free time, which does help, and I read books and such, but I think there is no substitute for just taking practice tests. I believe most people who only do Q or only V get worn out at test time, and 3 1/2 hours of test should be second nature, when youre gonna pay $250 a pop to take it again and again.
I hope I was of help.