Hii..i need to interface a cellphone with my arduino Mega 2560. Could you please let me know which of the pins of the K200i are Rx, Tx and gnd? Thanks.
normally google is your friend 
http://www.google.com/search?q=sonyericsson+k200i+pinout
I'm not sure of your phone model, but have a look at these pages from pinouts.ru:
Hope this helps 
Google is your friend?
Do you know if the K200i even has a serial connection?
After a quick google search I found this:
http://bestcelldist.com/sony_ericsson_k200i_serial_data_cable.html
Within 2 minutes I've found numerous websites with the information required on the pinout of the Sony Ericsson 12 pin connector. Not googling before asking here is just plain lazy.