i am selecting baud rate in serial monitor as 300 and making serial begin() as 9600 baudrate.
my program is if user enter character 'a' in serial monitor .some characters should be printed.so made transmitting baudrate as 300 and transmitting baudrate as 9600.is that right.
The baudrate for the serial monitor on the PC must be the same as the baudrate used by the Arduino.
If you write a short sketch that just prints a message to the serial monitor you can experiment with different baud rates and see what happens.
...R
If you were thinking TX and RX baud rates were independent, they're not.
I don't think 300 baud has been used for at least 20 years!
I don't quite recall what I did with my old 300 baud modem - it might just be hiding somewhere around, but it would be difficult. It had similar dimensions to (or was somewhat larger than) the original IBM PC and must have weighed at least 15 kg.
Good grief! Did it employ child-labor to transcribe the data and speak it across a standard telephone line? 
thanks for all your replies.i though that bitrate(baudrate) of tx and rx are independent of each other.but now i am clear that it is not.. thanks again
300 baud recently failed to work in a test sketch, It is on my todo list to understand why.