How do I extract data from a website after performing some actions on it?

I am building an app that will ask users for their login information for a website and can login using that information in the background and present the some specific data in a better manner. Most of the resources I could find only allow me to extract data from the website without performing actions in the website like logging in.

I am new to app development and was wondering the simplest way to solve this problem.(preferably using JavaScript)