Getting it to work
Getting this to work isn't hard at all. In fact it's really easy. You just need to either know the IP of the guy you are connecting to, or be able to port forward for hosting. But not everyone is used to stuff like this, so melee superstar Gekko has produced a couple of very handy guides to help you get started. Follow the links below, or look in the blue bar on the side.
Courtesy and rules
Playing against a human is different to playing the AI. Humans want a fun game and care about winning, which is very different from being programmed to not lose. Because of that the netmelee players usually stick to a set of pretty standard rules, to ensure that the game stays fun for both players. Not everyone needs to adhere to these rules, but if you aren't going to it's polite to let people know in advance. The rulesets are collected over on the Ultronomicon, but the most common ones are:
Contact and assistance
We are happy to help you with any melee issues on the forums of course. However, most melee players hang out on #uqm-arena on the Freenode IRC network, and can generally give you a hand in real time with connecting to other players and setting up your router. This is also where you'll generally find people to play against. Not everyone will be sitting at their computer at the very instance you log on, so there is generally a lead time of around 15 minutes between requesting a game and getting one.
Balance Mod & Guide
For tips and hints on how to play Netmelee, check out Shiver's PvP Guide. It's incomplete, but has a few good pointers.