first download the file in the link below.
Root Galaxy Light SGH-T989.zip
Mod Edit: link removed
Extract it once and you should have 3 files. Put your samsung galaxy light in download mode
press volume down+home+power. Start up odin on your pc. Plug in galaxy light while its in download mode. Choose pda. Look for the twrp recovery file. Choose it and click start. It will install recovery when done move the supersu.zip to your sdcard and boot into recovery mode volume up+home+power go to install and go to where the zip file was moved and install it. Now your rooted with a custom recovery. Congratulations.