List out all of the actions people can do that indicate they’re addressing their core motivation for signing up. There are a finite number of things people can do in your app and common sense is your friend here. If you do rideshare then it’s booking a cab, if you deliver food then it’s ordering dinner, if you offer courses then it’s doing a class.
This process isn’t always neat, you might end up with lots of maybes and that’s fine.
To narrow it down you want to think about how often people experience the problem your product solves. A good core action will line up with your best guess of how often they experience the problem.
Once you have your shortlist then you need to cross-reference everything with actual data.
The action you end up picking must correlate with long-term usage.
The whole point of understanding your core action is that when people do they’re more likely to stick around in the long run.
If you still have options at this point, always go with the one that’s easiest to understand. A decent metric that’s simple to grasp is better than something accurate with lots of footnotes.
You want the simplest core action that indicates that your product solves the problem people signed up to deal with.