HOSTING A MATCH FOR STEAM FRIENDS
Hosting a match is easy. Click MULTIPLAYER -> HOST MATCH and then configure your match in the following menu. Once you're done, your match will show as a server to your Steam friends in the game's server browser. Congratulations, you've just started your first listen server (aka the hosted match).
Besides TOXIKK's in-game server browser, your Steam friends can also join your hosted match through the Steam friends list or via invites using the Steam in-game overlay interface etc. You can also assign a password to make sure only friends knowing the password, can join your match.
HOSTING PUBLIC MATCHES
Besides the simple way of hosting matches described above, there are also other, more complex ways to host matches. E.g. if you want to host public matches on your listen server that reach people beyond your Steam friends list, your server needs to get published to all server browsers.
To make this happen, you need to configure your router (you know... dat thing that connects you to the interwebz) to FORWARD PORT 27015 UDP. And... the problem is, we can't really tell you how to do that, as there are bazillions of different routers out there. But for a start, THIS seems to be a useful general guide about forwarding ports with a huge list of available routers.
TROUBLESHOOTING FOR PUBLIC MATCH HOSTING
A - People can find my server but can't join it.
On some reagions on earth people have issues with SteamSockets. Either you ask one of your friends to host the match instead or you can disable SteamSockets in the serveroptions and forward additionally port 27015 UDP and 7777 UDP in your router.
B - The ping to my friends is really high and it's awefully lagging.
Its possible that your ISP has another router added between your pc and the rest of the world. This prevents the game from creating a direct connetion between the players. (and falls back to Steam relay (proxy) server) Again, ask one of your friends to host the match or ask your ISP to remove the second "security" router from your connection.