Create a Back end and fron end application using codeigniter 3

if this is your first app, suggest that you start with the content creation part of the blog - otherwise known as CRUD: Create, Read, Update, Delete.
take a look at the blog tutorial in the CI documentation to get started.

otherwise if you are set on creating users, authorization, etc - take a look at ion auth
https://github.com/benedmunds/CodeIgniter-Ion-Auth