This is most common problem every student face when he was in class 12 and preparing for competitive entrance exam. Right planning is required to make a balance between your competitive entrance exam with board exam. Now a days all most all competitive entrance exam is conducted by NTA and the recommended book of NTA is NCERT.
So start your preparation with NCERT text book. try to solve all question in the exercise and solve questions from NCERT exemplar. Do solve MCQ based question will help you in your entrance exam as well as board exam. Allocate equal time for board as well as competitive exam with effective revision of class 11.