why i think age 21 late for starting zero preparation for Sbi and too it is impossible for me to crack as i am weak student in all aspects i have no other career just have to crack exam but i feel its impossible being weak on studies to do it i really feel bad for family they have hopes i am useless

Hii!
Relax! Don't take too much stress. There comes a point in every one's life when everything seems worthless, specially you yourself feel worthlessness and it surrounds you with too much negativity, adding the expectations over it as a burden.
But, you need to calm yourself down.
Try making some basic changes to your routine. Don't procrastinate and just push yourself to do a little more. Stick to 5 min. Rule, wherein, you push yourself to do a work just for 5 min. when you don't feel like doing it at all. And remember, no one is useless or too weak in studies. You just have to recognise and trust your potential. If you think you can do it, you can. Everything depends on your will power. Forget expectations of your parents for some time and think.hiw you would feel if you succeed. Study for yourself and not others.
Get up before sunrise, sip on some warm water. Freshen up and do some yoga, or go out for a walk or do some exercise. Mediation will help you focus. You can try heartfulness meditation at afternoon or normal meditation in morning and evening. Sleep for 7 hours at night. Reduce the time you use phone for, and stay away from social media for most of the day. Don't compare yourself with others as you don't have same situations and amenities like them. Everyone has their own life. Take a balanced diet and don't procrastinate.
All this will help you focus in SBI exam preparation. You need to know the syllabus well, set a time table and start studying, take intervals in between. Drink enough water to keep yourself hydrated.
Now, I'll be sharing with you some links which will help you with information like preparation tips, important books, etc., Related to SBI exam.

https://www.google.com/amp/s/competition.careers360.com/articles/sbi-po-preparation-books/amp

https://www.google.com/amp/s/competition.careers360.com/articles/sbi-clerk-preparation-tips/amp

https://www.google.com/amp/s/competition.careers360.com/articles/sbi-po-preparation-tips/amp

Remember only you can change your condition, no one else can.

Hope this helps.
Good luck!