If you are a beginner use something like http://www.codecademy.com/ or https://www.khanacademy.org/computing/computer-programming - good for learning the basics

For more experience and getting to know the languages I recommend http://www.codewars.com/ - great for solving puzzles, very similar to Project Euler!!

Has anyone heard of https://keybase.io ? In my perception it does the same thing...