Hello there,
21years is not late, it is the ideal age for starting your SBI PO preparation. Do not stick with the memories of your past. You were lazy but that doesn't mean you cannot change yourself. Get up and start studying from now. If you work hard, you will definitely get good results.

Some suggestions about your preparation:
You should know your syllabus, and focus on your studies. Believe in yourself. Don't lose your confidence, be confident that you can do it. Keep aside the negative thoughts and concentrate on your studies. Complete your syllabus keeping enough time in hand for revision. Solve the previous year question papers. Give regular mock tests. You can only achieve your goal if you are confident enough.

Hope this answer helps you.
Good luck!!