Ok I am really new here.
How would you write a fanmail. Do you just write
"Hey!
I am writing to request your autograph. Thanks!
_______(name)"

Or do you write paragraph after paragraph describing your feelings and other stuff?

I found Sachin's address and phone number from quite a reliable source online, so here you go:

19-A, Perry Cross Road,
Bandra (West),
Mumbai – 400050,
Maharashtra

And:

(022) 26457576