How can I get information on a website after login?

I realized the design and the algorithm behind my mobile application but I have a problem.

I want to get some information of a webiste but I don't found the API of this website.

I'm lost during my research and I think that I have to parse the website ?

I want to get information after a login on the website of the user of my application.

I don't really skilled with web and english ^^

Thanks