You'll want all the MP DLCs in order to find games.
They're all free to download.
Resurgence Rebellion Earth Retaliation Reckoning
As for tips... stay with the team, help with objectives, use rockets on later rounds if needed? Oh, and watch out for the Collectors. They're pretty fun once you learn them but at first they can be a bit of a shock. --- "The act of treachery is an art, but the traitor himself is a piece of ****." - Mike Tyson
No. Firebase White's layout was changed and the Geth were updated. And Decoy was nerfed. It's still technically possible to use that strategy but it's not nearly as 'automatic' as it used to be.
The community's gotten a lot better (to some extent) since then, though, and there are also some ridiculously strong classes that have been released now (Geth Juggernaut for example).
Basically, get your feet wet on Unknown / Unknown / Silver and when you feel good with that just go for Unknown / Unknown / Gold. --- "The act of treachery is an art, but the traitor himself is a piece of ****." - Mike Tyson