Looks like you don't have any partitions on your sd card. That's why fdisk -l is coming up empty. You could also run the following to see partition info.
root@Pineapple:~# block info
You can make partitions with the fdisk command from your ssh console
root@Pineapple:~# fdisk /dev/sda
That should bring you to an interactive fdisk console where you can add some partitions. Probably one for your ext4 and a smaller one for a swap.
Then you can use the following to set up your new partitions
After all that I would run that block command