You could buy cultures but alhamdulilah it was much easier for us to buy a yogurt that had live cultures in it.
How to make yogurt in your crockpot:
Pour gallon of milk in crockpot
Heat on high for 2 hours
Turn off the crockpot and leave it alone for 3 hours
If you need tips for thicker yogurt: Tips for thicker homemade yogurt