for what you want to do I think it's not necessary a custom module, but if so, it's not so hard to do, but you can allways use some existing drupal modules mixed together to do what you want.
You will need:
Display Suite and
Login Redirect module
Basically I need Drupal only for user authorization. After a user
logged into the system I want to show him a table with some data.
For this purpose you can use Login Redirect module to redirect user to any page you want.
Now I need to code some PHP code in a blank page, how I can achieve
this in Drupal? I need to know where I must put my PHP code.
For this you can use Display Suite. Display suite or simply DS comes with a new a new Text Format: "Display Suite code". This text format allow you to add some custom code to your pages (javascript,php,etc)