Put an OS on the SSD, and then change the boot order to go for the SSD (The boot options are in the BIOS). Unfortunately, you'll need to reinstall all of your applications (make sure to take a backup of all your files, games, etc. first).
You'll need an external HDD for temporary storage. Literally any file will be unreachable after reinstalling the OS, due to the paths to reach those files being nonexistant in the reinstalled OS.