Confidence is key. The screening process is the hardest part as you might not be able to completely reveal your worth in a simple IQ test and a group discussion and hence your appearance and confidence is judged upon mainly. Be dressed smartly, as it gives you confidence and an overall good impression to the testing officers.
Honesty is the best policy. This is highly applicable in an SSB interview. If you speak the truth there are two possibilities, either you will be selected or you won't be. But if you lie there is only one, you won't be.
Originality is acknowledged and appreciated. Be yourself. If you try to be someone else and try to follow set patterns about how others approached their SSB then your chances go down significantly.
Express yourself out there. Show them why you deserve to be a part of the Indian Defence force. Don't be shy, make friends visit the city. You are judged upon these factors too during the interview.
For the psychological tests, read a book about the SSB process and aware yourself of the kinds of answer they desire. Your answers should reflect positivity always. Do not write negative stories.
GTO judges your capacity to think critically, leadership, and confidence in yourself. Lead from the front in command task. In a group task if you do not have a solution in mind, be ready to listen to others too. Take the initiative, that gives you some positive points.
Lastly, during the interview just be honest about yourself.
Stay fit for the SSB, a daily run would help build stamina for the GTO. Reading newspaper regularly would aware you of your surroundings and latest current affairs of the world.
A SSB is wonderful and must have experience in life. Enjoy it to the fullest, be prepared to learn. Believe in yourself. If you believe that you can do it, there is no reason why you can't.