As you mentioned that you have completed diploma in engineering, So you can apply for junior engineering exam.
If you have completed the graduation then you can apply for CGL exam of SSC .
SSC organizes its all exam at all Indian level.
For the preparation of these subject you need to work hard hard.
You should make time table for the proper study of SSC exam.
If possible you should join any leading SSC coaching of your region, otherwise you can study at home.
You should make grab on GS portion.
For this read newspaper and magazines.
You can read competitive magazine like pratiyogita Darpan.
Learn the tricks for the short soluction of math and reasoning questions.
Learn the English synonyms, antonyms and develop ability to understand correct English.
You should practice previous year question paper
As you mention that you are physically handicapped then you can give SSC exam till the age of 37 years.