Basically you just need to run some commands called "super su". As phones do not have keyboard, we have group those commands into a file called supersu.zip All you need to do is to run this file in your phone. This is called "Flashing" a zip file in your phone.
1) Boot into recover mode ( OFF then hold Power + Down )
2) Flash SuperSU zip file
http://download.chainfire.eu/370/SuperSU/UPDATE-SuperSU-v1.80.zip
DONE !!
If your device is NEW then you may NOT have a recover option yet. In that case, you will need to install this recovery function to your phone first. And you will have to do this from your PC ( the first time ).
1. Make sure your phone can be connected to your PC. ( the correct driver )
2. Copy superSU zip file to the phone
3. Download the Recovery function file to your PC
by Team Win : http://teamw.in/project/twrp2
by ClockMod : http://clockworkmod.com/rommanager
4. Download and extract Android SDK to your PC
http://developer.android.com/sdk/index.html
5. Looks for adb.exe and fastboot.exe commands. Usually at ...sdk/platform-tools
5a. Reboot phone to bootloader ( OFF then hold Power + Down )
5b. On PC, run CMD as Administrator. type fastboot devices to make sure the device is connected
5c. type
fastboot flash recovery openrecovery-twrp-2.6.3.3-mako.img
5d. type
fastboot boot openrecovery-twrp-2.6.3.3-mako.img
more info on fastboot
http://wiki.cyanogenmod.org/w/Doc:_fastboot_intro
6. this should boot into recovery mode. Now you can flash supersu zip file