
Here is how to do it:
Huzzah! You should be hosting a game and your friends/unknown people should find it.
If you want to find a server, just leave the host box unticked.
Occasionally Steam's NAT-punching doesn't work like a charm, which means you need to forward some ports on your home router if you want to host. You should always be able to find a game if someone is hosting one! This affects about 10-15% of users. Unfortunately, there is no way to tell if it works, other than by having a friend check out if they see your server.
Developers are known to enjoy beating people in 1vs1 and will be hosting a few games each day, see if you can catch them online!