To enable it you need ssh server, if you don't have it:
sudo apt-get install openssh-server
Then you have to change a row in your OpenSSH server configuration file:
sudo nano /etc/ssh/sshd_config
Replace the line:
Subsystem sftp /usr/lib/openssh/sftp-server
Subsystem sftp internal-sftp
NB: This works on Debian and all distributions based on it, like Ubuntu. The default port is 22 for ssh so you probably want it to be opened, if you want to access your sftp outside lan.