Hello STudent

The minimum height for police  inspector jobs is 170 cm for men and 157 CM for women. Chest must be 80 cm and by expansion 5 cm extra.weight must be 56 km for candidates and for Rajbanshi , Gorkhas & St candidates with 52 kg at least.

So if you are   underweight then try for gaining weight by taking healthy foods. Go to a dietitian and explain your interest of jobs he or she will chart out your food taking routine throughout a day. Have exercise regularly under a trainer. Read the syllabus of 10th standard for English , general knowledge and quantitative aptitude. For reasoning test get the help of many useful books of Arihant or any other renowned publishersa in the market. For related apps for reasoning tests download in  mobile.

Hope this helps.