Look, no offense. But anyone that doesn't understand this sort of basic networking concept is going to be at terrific risk if you start opening up ports. If you are going to proceed with this, exercise extreme caution, and do some reading.
You don't do anything at all to the FreeNAS. You need to instantiate a "port forwarding" rule at your gateway ingress (i.e., your "router" in most cases). This is something you set up on the gateway. Typically, you indicate if the rule applies to UDP, TCP, or both (in your case, you'll want TCP), you'll want to forward some ridiculous incoming port (say, take something above 20000 and you'll be fine...just pick a number, say 37771), and then you tell your gateway that port 37771 gets forwarded to your FreeNAS IP on port 22.
Then, from the outside, you'll SSH/SFTP connect to your home router's WAN IP on port 37771, and as far as your FreeNAS will be concerned, you're on port 22. Of course, your WAN IP is probably dynamic, so in order for this to be a viable long term solution, you're going to want some kind of dynamic DNS as well.
If you don't completely understand what I've just said without having to google things, then again, I want to reiterate, that opening ports/forwarding ports on your router does expose your internal network to outside shenanigans, and a networking novice is at significant risk if he does this. You will **AT A MINIMUM** want to completely disable password logins to SSH, and use only certificate-based authentication, which is additional complexity for you.
Proceed with caution, sir.
