Download ONIZUKA v 1.0.0 Free Version GTA V Online Mod Menu from the provided link.
Now Extract the downloaded rar file to your desktop.
After extraction start GTA V and launch storymode.
Now open Onizuka.exe as administrator and rest things it will do automatically.
Go back to GTA V and try opening mod menu by pressing F9 key.
If mod menu starts success open GTA online.