This is really straightforward (once you know how!)
First, make sure your system is set up to send emails under System Settings > Email. For reliable sending of email, I strongly recommend that you use a service such as Sendgrid (free for up to 10k emails per month, I think).
Then, under your data point details, scroll down to the very bottom and under 'event detector', add a detector of type 'high limit' (and 'low limit', if you like). The fields should be self-explanatory, but check the 'use reset limit' box if you want to receive another alarm when the point returns to normal.
Save the point, and finally, browse to your 'event handlers' page, where you will see your point and alarm/s listed under 'point event detectors'. Here you can add recipients and customise the email further.