Installing to this project is quite easy.
Make sure you have node installed and run:
$ npm install
Then your most useful command while contributing will be:
$ npm test
If you work on a bug it's recommended to write a test that replicates your issue and work on your fix, making sure you don't break other tests.
You can learn how from this free series How to Contribute to an Open Source Project on GitHub by @kentcdodds.
If you want to make an improvement to the library, please open an issue first. If you want to fix something, I'd prefer if you open an issue first but I'm quite open, as long as you provide a test for your fix and don't break existing tests.
If you just want to contribute, open the list of issues and pick one to investigate.