The only logical and non-complicated way to do this is phishing the login page. Even now with anti phishing browsers that can tell if the site is a legit page or not, its still difficult to do unless you know what you are doing.

90% of the questions you have about hacking are going to be dead end if your initial request is "how to hack"

There is alot you need to read up on first before doing so, with that start at the basics and work your way up then form legitimate questions.