You probably already have make installed. For example, OS X and many other POSIX systems come with make, but some versions of Linux do not include it by default and will require you to install it. If you’re on Ubuntu, you can run sudo apt install make to get it.
You can install Reach by running:
$ curl https://raw.githubusercontent.com/reach-sh/reach-lang/master/reach -o reach ; chmod +x reach
in your project repository. You can copy this file to other repositories or move it to a directory in your PATH, like ~/bin.
If you’re using Windows, consult the guide to using Reach on Windows.