I have completed 10th from kerala state board(scert) and currently studying in 12th. for 10th we follow grading system A+,A,B+,B....instead of marks.I got A+ in all subjects that is overall 90%. so how should i fill the marks obtained coloumn in the application because we dont know 10th marks

Hii..
As you ask about how to fill the application form marks column ... so every great have there marks system as:
91-100.   A1
81-90.     A2
71-80.      B1
61-70.          B2
And so on you can use this system to calculate your marks if you have any confusion about this so you can go through this link and you will get your answer and I hope you will clear your doubt..
https://mycbseguide.com/blog/cgpa-calculator-how-to-get-marks-in-percentage/
All the best!!