In law school, the first thing taught is "to lie" but said in Latin, what is it?

If you're looking for the Latin for "to lie", meaning "to deceive", then the verb is mentiri.

However, I'm not a lawyer, so I'm not 100% sure that this is the specific term that you were looking for.

You might find the website Legum.org helpful, as it contains a long list of Latin terms used in the Law. Hope this helps a bit!