First you need to access to developer options on your device.
For this you need to go to settings / security and there you have to activate unknown sources
This will let you install aplications that you download from your browser (be very carefull with the apps that you install).
Then open your browser on your android device and go to KingRoot.net
There is a button in Chinese but don´t worry is nothing bad. Press the button with the android logo, you will se this dialog:
Then install it, open it and click on root
There you will se an aplication called King User that is instead of SuperSu
If you want to check if your device is correctly rooted go to the playstore and download RootChecker