Is there a website where you can collaborate for free?

Yes, indeed you can use Github Build software better, together or Bitbucket | The Git solution for professional teams and many others such as CodePen, But if you are asking about pair programming, there are also tons of them out there; most IDE have got it. https://codeshare.io/ is also great!

Yes.The best known is Github, which is free for projects that are open to the public.It's easy to set up a repository if you're just tinkering.

There are websites where you can work for free.As an example, you can think of GitHub.Go ahead and clone a project that interests you.