The short answer to this is choose the current stable version of bootstrap css framework and use the responsive grid if your developing the templates or html yourself.
If you are using something like wordpress, find a theme that is responsive or use the _S there and customize it.
If you want to learn some new things like SASS, then you might want to have a look at bourbon.io I am going this route. There are some add on libraries, but things are in a state of flux right now. They are moving to a new version, so the supporting libraries may not work with the latest master branch. You will have to pick the stable version and go through the git commits to see which versions of the supporting libraries work with the stable version.
Its a bit of work, but its easier to customize things with SASS and the css is responsive and very tiny in size.