In order:
Your gamertag can be put in using the control panel for your forum account, under the "Edit Profile" link, in the "Additional Information" section.
UPnP (or, fully, Universal Plug-n-Play) is configured through your router, and in many situations not worth the extra security risk of leaving on/open.
Both. Selecting the "Quick Match" option, in the multiplayer menu, will throw you into the first handy game. Using "Custom Match" will give you a list of available games, their host, and the current/max player count (I think the current count doesn't show reserved slots being used by invited friends, but I'm not sure on that). There are various ways to filter the output, by things like frag (death) counts, match type, and whether the match is a player match or a ranked one (ranked matches count towards TrueSkill™ ratings, and are generally more strict than player ones, limiting what settings the match creator can change). Some of the achievements can be done in either mode (player/ranked), but others specifically require ranked matches, due mostly to that decreased flexibility I mentioned, above.