The easiest way to do it, is to join a public multiplayer server and play together on there. Downside is that you might run into trolls.
If you want to play together with someone in the same house (more specifically: same LAN), then you can also easily host your own server.
It is, of course, also possible to host a server publicly. And it's possible to allow someone to basically connect to your LAN, so that it is as if they were within your LAN (with a so-called "VPN"). But these last two are somewhat more complex.
Which one are you looking to do?