I have passed 10^th grade with A1 in all subjects.I want to qualify in the KVPY 2015.Do i have any chance if i start preparing now?If so how should i prepare?

Very congrats for Class 10^th credentials first.
You should start today onwards, for KVPY 2016, SA level, in which entire syllabus of class 11^th and 12^th would be covered.
Exam in sept 2015 for KVPY would be typical for you this time, because you are unaware with class 12^th syllabus.