I may have assumed too much knowledge too. The actual software you'd use to host the SSH server comes standard on Linux systems, but can be installed on windows and OS X (if you must). It's called OpenSSH. The Android apps for interfacing with it would be AndFTP (at minimum) and Termius (optional but handy as this will also let you have command line access and also can set up SSH tunnels which can be thought of as a poor man's VPN, and are very handy for pivoting through an SSH server perhaps to a file server that itself isn't even on the routed Internet).