Hi

Yes, you will have to give examination of all the subject.

This is because,

A private candidate is one who fails in his board examination and then repeats and appears for board examination again and when he repeat he has to give examination of all the subjects.

So, as a private candidate, you will have to give examination for all the subject .

And yes be sincere towards your studies this time , follow a time table, go through previous year papers will help you to understand the types of questions which are asked in exam, you will understand the important topics which are covered in the exam by practising them so that you can accordingly prepare for the exam and you will also understand the mistakes you did in your last board examination so that you don't repeat them.

Good luck, prepare well:)