Login to Website

I am inquiring about the ability to log-in(as a member) to a website that I do not own but would like to make an app to access information after logging in. It is for a company that I work for and dont know how to go about doing this.

Is there a way to reference the url? Can I directly link my project to the url? How do I access the information on the site after logging in?

Any help on this subject would be great, and I would like help ASAP, this is a bit of a time sensitive project.