Is slander a criminal offence, what are the common penalties, and what evidence do you need to successfully get a conviction?

Normally a civil issue where the damaged party sues for actual damages.

If you can give more info about your specific case.

As noted, it's a civil, not criminal matter. But that makes it no less illegal. To win a case, you have to prove that was was said about you was malicious, that you were damaged by it and that the person who said the things you object to knew they were untrue.

Why do I think this is homework?