How to lock notes app
Welcome to iMore! There is no way to lock an app. Some 3rd party apps have log ins. You have 2 choices.
1. Move your notes to an app you can lock with a log in. I use Box for private Notes. It has a Notes feature. There's others.
2. Just keep your phone locked and don't allow anyone to use it. If you let someone use say one app, you can contain them in that one app using Guided Access in Accessibility.