There isn't much you need to actually know about "G.I. Joe: Retaliation." There are no books to read, or mythology to know and hell, you probably didn't even have to see its predecessor, "G.I. Joe: Rise Of The Cobra." But one thing you should probably know as you make your decision to buy a ticket is this: the movie has ninjas. We know, ticket bought, right?
Because no matter how good or (likely) not good the sequel is, we'll get ninjas, and that's pretty awesome. But it seems Paramount wants to give away that whole big money scene for free, so here's a pretty generous 4 minute clip of the zomg-they're-fighting-on-a-fucking-mountain scene. This seems like the reason you would actually pay for a 3D ticket and see it on the big screen, but here it is to watch on your computer or mobile phone or whatever because marketing. After that, you can watch a brief featurette which confirms that the movie is a giant playbox for a bunch of adult dudes.
"G.I. Joe: Retaliation" makes toy stores happy on March 29th.