So how should I start? Should I first go through the math related manhatten books to get an insight into everything and then continue with the OG questions?I am bit confused by the first practice test. Should I just take it right away before opening any book? Or should I do it after getting the basic skills and before solving any OG problem?

Please take diagnostic test from any of the test prep companies(

Veritas,Manhattan) and understand where you stand..That should help...

You can also try GMATCLUB Diagnostic test for Math...It can be a good indicator of your current skill levels but I think a 1 complete test will give you a good overall picture.Take the tests with only stipulated breaks and in one go to get the most out of it.

The next thing is, that I am not sure if I need all the math stuff (topics in Manhatten books) or are there sections which are only involved in the very hard problems? (750+ or more)

Because I am "only" aiming for a low 700 score.

Don't get into that mode, but yes a solid grasp on Number Properties, Geometry,Statistics and Algebra basics will give enough confidence to tackle questions of any higher difficulty levels..

Some useful links

gmat-prep-software-analysis-and-what-if-scenarios-146146.htmlall-you-need-for-quant-140445.html#p1130136This should get you started.

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“If you can't fly then run, if you can't run then walk, if you can't walk then crawl, but whatever you do you have to keep moving forward.”