Apple Account

Hello,

I have bought an iPad Pro and set up the Apple ID and password like someone else's. Now I wish to have my own Apple ID and password. When I tried changing the settings from the Apple ID account page it changed the account information on my friend's iPhone, that is it changed the Apple ID and password for her too.

I wish to have my iPad as a separate device that is have an account of my own. Can anyone help me with this please?

Thank you very much