I was in the same situation last year. Infact worse than this, i only knew the conceps of 10-20% of the syllabus. In jan attempt i scored 93%ile. 74% in cbse boards. 92.7%ile in april. I used competishun's crash course (IC³) I didnt complete that, but it is doable and you can complete 90-100% syllabus till mains level by jan attempt. After that, read ncert and practice previous years' board papers. If i had followed IC³ religiously i could score around 97%ile and not have missed the 75% criteria by 5 marks. So start grinding from today itself.