Normally my honest advice would be to open up your game engine of choice and just start messing around in it. Learn how it works, learn what the tools you have at your disposal are. Look up a video on how to create a mechanic or material or something and go through it.
But it's a bit late for that, so...
Keep what you contribute small, simple and manageable. We're not building the next JRPG or Assassin's Creed here. and you don't have a ton of time.
And finally good luck.