You have two options...
Change the current working directory to a path of your choice and enter...
git clone https://github.com/cc65/cc65.git
This builds both the cc65 binaries (located in the 'bin' subdirectory) and the cc65 libraries. Now cc65 is fully functional without further steps.
If you want to be able to run the cc65 binaries without providing their path you may additionally enter sudo make avail. This creates symbolic links to the binaries in '/usr/local/bin'.
Install one of the pre-built packages for RPM based systems (OpenSUSE) or DEB based systems (Debian) from the openSUSE Build Service download page for cc65.
Download the current cc65 snapshot and unzip it to a path of your choice that doesn't contain spaces (let's presume 'c:\cc65' here). Now cc65 is fully functional without further steps.
If you want to be able to run the cc65 binaries without providing their path you may additionally add 'c:\cc65\bin' to your 'Path' environment variable.
Note: If you have used cc65 before then you need to make sure to remove all environment variables related to cc65 (like CC65_HOME).