1. Unlock Bootloader
2. Put the su.zip file on the phones sdcard, DO NOT EXTRACT IT, turn off Fast Boot in the settings (settings>power) and turn off phone, hold volume down and power to go into the bootloader if your phone is still connected to your computer it should say "fastboot USB" on your phone.
3. Flash CWM (fastboot flash recovery nameofrecovery.img) ("Recovery" will now appear in the bootloader after a restart, if not powerdown the phone and enter the bootloader again),
4. Flash su.zip file from inside recovery (CWM) (Bootloader>Recovery>"Install zip from sdcard">"Choose zip from sdcard">select the su.zip and it will flash, exit and restart the phone and you should have root.
If you're going to flash a custom ROM you don't have to bother with root as the ROMs are pre-rooted and just follow the instructions on how to flash your desired ROM after you have unlocked the bootloader and flashed CWM recovery.