I'm running OS X Sierra and I'm connected to an Apple AirPort Time Capsule. I cannot, for the life of me, find how to properly port forward the stupid computer. I have a good amount of programming experience, but know hardly anything of networking. I have a functional Node Express server that I can run and access through localhost on my machine, but I want to open it to some friends. I recall when port forwarding for a Minecraft server a long time ago on Windows, and had a Linksys router, all I had to do was type in the router's local IPv4 in the browser and port forward using Linksys controls. I have my server listening on port 8080, what to do to make it available at an external IP address? Internet basics were briefly explained to me recently, but an additional explanation of how local networks work and how they interact with other networks would be greatly appreciated!
The AirPort products of apple aren't configured via a web interface like most routers, you will have to use the "AirPort Utility" application on your Mac. When you select your Airport Time Capsule from the first screen of the utility, go to Network and click the plus under Port Settings. You should be able to go from there. Depending on how long you want to keep this setup you might want to ensure your Mac keeps the same IP in your network.
A quick google search lead me here as a guide with images: https://owlr.uservoice.com/knowledgebase/articles/875730-how-to-set-up-port-forwarding-on-an-airport-time-c
I'm wondering why you weren't able to find this yourself? Your question sounds a bit like you expected it to be somewhere in the Settings of your Mac, so just to make clear: You configure the Router for port forwarding, in your case your Timecapsule. This could also explain why you got downvoted (I assume people thought you were just lazy or something... :) ). Anyways, I hope this helps you out (and I hope Apple didn't by now remove this configuration feature at all... I haven't used the AirPort Utility in quite a while...)