First of all, you need to stop worrying and realise that you really donot have an option other than studying. So why not better get going?
So here is some of my advice
You should start your preparation chapterwise. Since you’ll be writing boards too, I would suggest you to be a little more methodological in your approach. Keep note of the chapterwise mark distribution for each subject and begin your preparation likewise.You have sufficient time.
Pick up a chapter, say Haloalkanes and Haloarenes in Chemistry, firstly read the NCERT, then read your notes and thoroughly practice NCERT questions. Once you are thorough with NCERT for that chapter, practice previous years’ questions from sample papers or previous years’ paper. (Look for them online, you can find them easily)
Repeat this for every chapter. Complete your syllabus well, know the concepts, learn the formulas.
Once you are through with your syllabus, I’d suggest you to take 3 hour tests or solve sample papers provided by the CBSE. Also, do solve previous years papers, donot neglect those. This should give you ample practice.