If you don't know then refer to this post http://forum.xda-developers.com/show....php?t=1527159
WARNING: IT WILL WIPE YOUR PHONE DATA SO MAKE BACKUP
1. Install Flashboot and Fastboot drivers of Flashtool (setup file will be on C:\Flashtool\drivers or similar place).
2. Turn on your phone, and connect with usb debugging on.
3. Go to Plugins-> Bootloader Unlock option, start it.
4. Now it will ask to disconnect your phone and reboot in FLASHMODE.
5. Again reboot your phone in FASTBOOT mode.
6. It will show a window with your IMEI number, insert your unlock key and click "unlock".
7. The device will reboot and you are done!
You can get into flashmode by pressing specific hardware key and connecting phone to pc via USB.
It will glow GREEN LIGHT on led if done correctly.
Back Button (X8, X10, X10 mini, X10 mini pro, Play, Arc, Arc S, Neo, Neo V, Pro)
Volume Down Button (Xperia U,P,S,Sola)
You get into fastboot mode by turning off your phone, then pressing the fastboot mode key button, and connecting the USB to your phone. Done correctly and you should see a BLUE LIGHT on the power button.
Menu Botton (Xperia Arc, Arc S, Neo, Neo V, Pro)
Volume Up Botton (Xperia U,P,S,Sola)