I am not sure about other background information about you, but based on details you gave, I would say you need to work really really hard if you have not done any practice before.
GMAT tests basic concepts from high school, but familiarity with questions is very important because questions are tricky, and have many ways to trap novice exam taker. To achieve 700+, you need to be able to get past a good number of traps and solve harder questions with consistecy, which can be achieved by doing more and more practice.
I don't intend to discourage you at all, but rather the intention is to motivate you to use these 20 days and work hard to go above and beyond. Hope this helps.
Some other posts by me...
My GMAT Journey
Timer Tool for Google spreadsheet
Quant Flash Cards/Pages
700 (Q:49 V:35) - Thanks GMATClub - Test Center experience