$ sudo swapon /dev/sda1If it works ('sudo swapon -s' shows it), you can run this command (again, modify the partition to fit your case):
$ echo '/dev/sda1 swap swap defaults 0 0' | sudo tee -a /etc/fstab
The line appended to /etc/fstab, "/dev/sda1 swap swap defaults 0 0", asks for the activation of the swap at init.