So you have downloaded MegMek, played against the bot, and now you are ready to try your hand against a real live human opponent. So where should you go to find other players? There are several options. Many people just organize pick-up games, campaigns, and tournaments via various Classic Battletech forums. We maintain our own forums on this site. Another popular forum is the MegaMek sub-forum at the official battletech forums. You could also try the forums at Solaris 7 , Dropship Command, or Lords of the Battlefield.
Many people also play MegaMek on one of the various MekWars servers. MekWars is an online campaign “wrapper” for MegaMek that allows players to manage a set of meks, fight out battles, collect salvage, and so forth. The currently available MekWars Servers are listed in the link above. Another option is to become involved with the Fan Game Councils. These game councils use a play-by-post method of playing out a grand strategic game and MegaMek to resolve specific battles. Net Battletech Classic offers a similar combination of grand strategy combined with MegaMek games.
Finally, many of us just develop a long list of IM contacts from these various sources that we can hit up for either pick-up or regular games. There is also an IRC channel (#battletech) on irc.esper.net, where you can chat and set up games.