Can I use the word "discover" only when I found something for the first being?
Yes you are correct. The word "discover" is usually used to mean that you found information, a place, or an object, especially for the first time. It can also mean that you found something by accident while looking for something else, although this still carries the nuance of it being the first time. Ex: While going through out letters I discovered he lied about being accepted to Harvard. Ex: Scientists have discovered how to predict an earthquake. Ex: I recently discovered a lovely cafe near my office. Ex: I wonder who first discovered fire.