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Here u go to root your device easily .. jus a two min process http://forum.xda-developers.com/showthread.php?t=2335992
.. also u need to install cwm to flash any custom roms .. to install cwm http://forum.xda-developers.com/showthread.php?t=2020895 .. do not flash a custom kernel !! it ll brick your device, if u have unlocked ur bootloader then can flash custom kernel !! unlocking boatloader voids warranty !!
custom roms for xperia p
1.http://forum.xda-developers.com/showthread.php?t=2258058
2.http://forum.xda-developers.com/showthread.php?t=2422819
3.http://forum.xda-developers.com/showthread.php?t=2391858
there are also so many roms available ..
jus search here
Everything you need to know and find is here