how can add sim900(sim900.h) header to keil. it is possible to add
In your Keil installation, there is a Getting Started Guide - search for "GS51"
uVision IDE manual: http://www.keil.com/support/man/docs/uv4/
C51 Compiler manual: http://www.keil.com/support/man/docs/c51/
Worked examples here: http://www.keil.com/support/man/docs/uv4/uv4_examples.htm
There are books here: http://www.keil.com/books/8051books.asp