For Officer Training Academy entrance exam, To clear OTA you have to be good in English and GK section and there is a bit elementary maths as well.

The first step is to know the updated syllabus and paper pattern and allocate your sections and time accordingly.

Solve previous year sample papers and even after you preparation you can revise through mock tests.

Prepare and read everyday GK because mugging up it at last moment is not an easy task.

Some of the books you can refer are:

• Combined Defence Services Examination by Jain 
• Combined Defence Services, Solved Papers by Lal
• The Pearson Guide to the Combined Defence Services Examination by Edgar Thorpe
• Arihant CDS Entrance Examination (Self Study Guide) by Brijesh Tripathi 

Good Luck!