Just tell the reason why do you want it, the purpose actually. If you need the format, it is "The name of the student" of "the class in which he is in" of "the school name" is the student for the year "2010-2011".. And latter, at the right bottom corner, the signature of the Headmistress/Headmaster.
i want Bonafide Certificate of my son
It would be easier to find an example if you could provide some more information. For example, who are you writing to? Generally, though, a letter of this kind is formal - so you would begin 'Dear Mr/Ms X' and state clearly what you want, eg 'I am writing to request a bona fide certificate for...' You could also look at some of the other Blurtit answers to questions similar to yours (on the right of this answer box) as they may have useful ideas too.