Use も when you're adding more information to already established information. It may be used alone when marking subjects and objects, and can follow other particles (like に, で, and へ). Put it this way: if は clears the existing data and replaces it, も adds additional data on top of what's already been said without clearing out anything.

Note what was deleted and why. Here you add yourself to the understood subject.

The particle も is easy enough, so we won't waste any more time with it.