After the watergate scandal Nixon was found guilty of perjury, coverups, obstruction to justice and several other crimes. Why wasn't he convicted like many others involved in this? Can a president even be imprisoned in the first place?

A president can be imprisoned after he is impeached, but Nixon was never impeached. Once it became clear that he was going to be impeached, he resigned. His successor, President Ford, pardoned Nixon really quickly after he became president of any of the crimes he may have committed so that's why he wasn't convicted or put on trial.