The break in led to Nixon's resignation because Nixon tried to use his power as president to cover up what had happened.
It does not appear that Nixon personally had anything to do with ordering the break in. He did not know it was going to happen. But once it did happen, he thought it could hurt him politically.
Because he thought this, he started using his power as president to try to prevent the facts about the case from becoming known. This means that he was obstructing justice, which is a crime.
When this got found out, he resigned so he wouldn't be impeached.