Well, starting out is going to be rough. The Sins online community isn't very large and many of the top players have been playing regularly since the game came out. Most of them know each other pretty well and will kick newbs out of games occasionally for other veterans. You may also be accused of being a smurf, which is a veteran player using a new name so that he will be mistaken for a newb and unbalance the teams.

Once you get a game, expect to lose for a while. You can improve your game a bit in advance by watching some online replays, as well as some of the threads in the strategy forum. You'll want to at least hold out long enough for the rest of your team to hopefully get an advantage against their opponents.

Also of note is that a new patch will hopefully be coming out Tuesday. This can be a good time to start playing because depending on the balance changes the "metagame" might be altered, and thus people will be experimenting more trying to create or refine new strategies.

It's an awesome game for online multiplayer.  You're liable to find newer players on the regular Sins/Entrenchment server, but it's mostly people playing comp stomps with their friends.  The real action is on the Diplomacy server, but like Goafan said, most of the players are very experienced with many being pro.  However, there are still some newer people around.  If you're serious about playing it PvP, you really should master the game against the AI first and get comfortable with all three races, the tech trees, and all of the game mechanics.

Hopefully it will be easier for new people to get into it when the Rebellion expansion comes out, if it brings hundreds of new people online.

Yes and no.

Rebellion is going to be sold as stand alone but as far as we know it will have everything diplomacy has with a lot of stuff added.

You could treat it as both really.

I'd say do some PvP practice with friends or if you play vanilla sins just look for Berek Thunderfist, Sindri, or Hambo3, we usually let random people join as long as they're not dicks, we usually play with at least 3 AI and lately we've been turning them to hard but depending on how fast the game goes it sometimes turns into a friendly game of PvP