Hi NA_JS,
There are certainly plenty of online resources that you can use for free during your studies, but your studies would almost certainly be more organized (and effective) if you invest in some GMAT study materials. Considering the long-term value of having a high GMAT Score (re: improved application(s), a chance at getting into better Schools, Scholarship possibilities, a potential factor in job interviews, etc.) the costs associated with studying for the Exam are rather low.
In answer to your immediate questions, the primary Official Guide from any of the last several years would be a good investment (100s of retired Official questions along with content lessons) and using the 2 free GMAC CATs is a must. While the most realistic CATs available are the 6 from GMAC, retaking a CAT that you've already taken is NOT a realistic way to assess your skills. Seeing even a few 'repeat' questions can 'throw off' the Scoring Algorithm and impact your pacing, energy levels, fatigue, etc. (meaning that they would likely all appear to be better than they normally would be). Thankfully, the CATs from Kaplan,
MGMAT and Veritas are all 'close enough' to the real thing that they will provide you with a relatively realistic score assessment (assuming that you correctly take the CAT in a realistic fashion).
Before I can offer you any additional advice for your studies, it would help if you could provide a bit more information on how you've been studying and your goals:
Studies:
1) How long have you studied so far?
2) What study materials have you used so far?
3) How have you scored on EACH of your CATs (including the Quant and Verbal Scaled Scores for EACH)?
Goals:
4) What is your goal score?
5) When are you planning to take the GMAT?
6) When are you planning to apply to Business School?
7) What Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich