First, it is best not to publicly point out that it is the first time for the other party to steal, because it is a roommate, so we should give the other party a chance to turn over a new leaf. At this time, if you find your roommate stealing, you can remind the other party not to continue this behavior in some ways. It's best not to publicly point out each other's theft, so it's not good for everyone to know.
Second, talk to each other privately. Although you don't publicly point out the other party's theft, you should let the other party know your mistake, otherwise it will happen again next time. You can communicate with each other alone and tell each other what the consequences of stealing are.
Third, if the other party commits another crime, you can capture it without worrying about the feelings of the other party. After a communication, I believe that the other party will not steal again. But some people won't listen, and it is inevitable to come again next time. If you find that the other party refuses to change their mind, you can publicly point out the other party's theft in the following time, hand it over to the school or call the police directly to solve the problem.
In any case, theft is a bad behavior, which not only harms people, but also harms themselves. Therefore, when you find your roommate stealing, you should first think about saving each other. If not, take other measures to stop the other party's theft.