In an interview with Entertainment weekly, James Gunn, director of Guardians of the Galaxy, talked plot.
Gunn to Entertainment Weekly
[Guardians of the Galaxy](movie:424073) will "definitely be connected to [The Avengers 3](movie:738027)."
He also confirmed the Mad Titan, Thanos's appearance in Guardians as they will be using Motion Capture to give him life. He also called Thanos 'the head of the snake' as we are already aware that Thanos is giving the orders to Ronan and Nebula and, at one time, Gamora too.
About Gamora and Nebula's relationship, Gunn said
"Thanos slaughtered her family and Nebula's, then raised them together as siblings. They were bio-engineered to become living weapons. So she's really good at killing people," Gunn said about Gamora.
On top of that Ronan was involved in killing Drax's wife and son, so there's definitely a showdown happening there!
Previously it was mentioned that the character Yondu (played by Walking Dead's Michael Rooker) raised Starlord.
In the EW article, Gunn said,
"[Star-Lord is] raised in a group of thieves and smugglers called the Ravagers. He's good with the ladies and a little bit of a dick and very lucky."