There are several apps available on the AppStore to secure your data. I've seen apps to just lock your notes, to create a folder to have other types of files in it and overall password apps to keep all data private.
What I use is an app called "1Password" (regularly $17.99 but on sale for $8.99). With it, I can make secure notes, logins and lists of all the passwords I use. It's accessible via one main master password. If you set it up to [securely] store you data online (in a cloud account such as Dropbox), you can access the information via your computer's web browser. It's all secure, but accessible.
In addition, for more $$, you can buy a server program to install on your PC or Mac that makes it easier to access via computer (easier as in less steps than accessing via the web).
As you probably can tell, I think this is the best secure note/password app out there. But, I'm biased cause I like it - so I hope others will be along to give you their opinions. Also, a search of the AppStore using the term "lock note" gave me 416 results, so I'm sure there are plenty of alternatives out there.
Good luck and happy searching.
Marilyn