Well by itself a .scss file does nothing. It has to be “compiled” into a .css file and then included like you would normally include a css-file into your index.html (link tag). You need a build tool like grunt or gulp (or even something like webpack) to have it compiled for you. That is the easiest way to get the css into your app.
If you really want to be able to just do “import ‘./style.css’” to use your styles you will need to look into webpack and its loaders, in this case style-loader, css-loader and sass-loader.
EDIT: Ok, actually you don’t NEED a build tool to compile it. You could do that by hand but a build tool automates the process and can actually “watch” files for changes and autocompile and stuff.
EDIT2: Have a look at https://github.com/webpack-contrib/sass-loader if webpack piques your interest