With few exceptions, the stock apps that come with the iPad cannot be individually locked, so anybody you allow to use your iPad will have full access to your contacts, photos, etc. Mail accounts are secured by their own log in passwords and can't be viewed without logging in. With iOS 9.3, individual notes in the notes app can be password protected.
There are third party apps that can give password protection to photos stored within the app, for example.
Unlike a computer, you can't have several user accounts, each with their own apps, passwords, etc. On the iPad. There is only one exception for the education market which allows this, however, but it's not available to the regular user.