In order to hack into someones account, all you need is their Login & Password. That simple.
If you don't have their Login, then you really don't have much of a chance to be successful.
If you have their login (email address) you can do forget password and if it allows you to reset it using security questions then you would need to know that person well enough to answer these security questions.
Otherwise you can use the email address as the login, and quickly code a script to generate passwords and try to authenticate each time a new password is generated. I wouldn't doubt that the account or IP address used to do this will be locked due to basic security reasons.
Best of luck, provide more information about what your intentions are, how many accounts you're really trying to hack into, what the big picture is and maybe you could get a better answer. What I provided was just basic common sense that anyone with a computer should know however not everyone has common sense hehe.