The low score is mostly because of the severe penalty for not completing the test. For example, if you had just randomly selected answers to the last 10 questions, your score in the quant would have been 50th percentile(my best guess). Pacing is really important on the GMAT, and it is important to have a good gauge of one's ability when it comes to tackling the harder questions. Often, it is best to let go the harder questions. This is more of an issue for students scoring close to the average score, as you raise your level there is less room to let go the harder questions.
I wouldn't worry too much about the low quant score in your first diagnostic test. This is fairly common and is partly because most students are not used to the adaptive nature of the test, and on top of that the average difficulty of the questions is GMATPrep is fairly high and it is easy to get sucked in to the hard questions. For example, you can miss six questions in the quant(in GMATPrep only) and still get a Q51.
For now, I would recommend you to work on foundations and make sure you are very comfortable with the subject matter. Certainly use the Official Guide questions to get a hang of the GMAT style and common traps. You will make a lot of mistakes and this is a normal process for most students. At the same time, also start doing timed practice to add the element of pressure and get in the habit of getting to the end of the test, even if it means letting go a few questions here and there. For this I recommend using the GMATPrep Question Pack 1, and create mock tests. You can see this link where I describe how to do this: http://www.gmatquantum.com/faq/gmatprep ... -of-t.html
And reserve the adaptive tests in GMATPrep and GMATPrep Exam Pack 1 for later once you have gone through this initial process.
Free Video Explanations: OFFICIAL GUIDE FOR GMAT REVIEW 2015.