The team we know we will see on the screen will be a 5-member squad composed of a pan-galactic group of loners and one-offs who will come together, apparently under some very shady circumstances, and, eventually, become a unit that will save the galactic day. Those members, are left to right, Drax the Destroyer, Gamora, Peter Quill, aka Star-Lord, Groot and Rocket Racoon. Let's do a little exploring into each of these unique characters.
The leader of the team, Peter Quill, is half-human, half-Spartax and all arrogance. Quill's most recent origin story sees J'son, the emperor of the Spartoi empire, crash land on Earth in the middle of nowhere and fall in love with a nice young lady named Meredith who helps him back to health while he fixes his ship. Once his ship is space-ready, J'son blasts off, leaving Meredith behind. A flash-forward later, we see a young and full-of-piss-and-vinegar Peter who is a spitting image of J'son. Not long after the Badoon, galactic baddies, come to terminate Meredith, who they believe to still be pregnant, so they can kill the air to the Spartax empire. Meredith bites the dust; Peter survives and is given his father's gun, which people believe to be a toy; Peter grows up angry, but still grows up with outer space in his blood and that, of course, is where we see him.
Quill travels space, in spite of his father's request to come "home" and be the heir to the empire, womanizing and getting into all sorts of shenanigans. He eventually comes into conflict with The Fallen One, a former herald of Galactus, who means to starve Galactus by doing his own galaxy wide dance of destruction. Quill learns the hard way that he doesn't have the firepower to hurt The Fallen One so, after some rest and rehab, he decides to use The Fallen One's destructive ways against him. Quill destroys a mining colony on the outskirts of the Kree empire with the knowledge that the energy released from the destruction of planet causes great pain to the former Herald. The Fallen One is beaten but the cost is high: thousands of colonists are slaughtered. Star-Lord turns himself over to the Kree and is sentenced to the Kyln, the galactic maximum security prison which we also know we will see on screen. Here Quill abandons the title of Star-Lord and receives some fancy cybernetic updates to help him recover from the damage done to him while defeating The Fallen One.
Quill's term in the Kyln is cut short by the events of the Annihilation during which he aids Nova, Drax the Destroyer and Ronan the Accuser in the ultimate defeat of Annhilus and his Annihilation Wave. Afterwards Quill finds himself in high favor of Ronan, now the leader of the Kree empire, and is employed as a military strategist. It is during this time that the Phalanx make their move on an already weakened Kree and Quill becomes part of the resistance. His cybernetic dodads are removed by Kree scientists and he is outfitted with what will become his more well-known and, apparently, on-screen look.
During the events of Annihilation:Conquest (the war with the Phalanx), Quill reclaims the title of Star-Lord and meets those who will eventually become part of his team of Guardians.
Star-Lord and the Guardians come into conflict with no shortage of space villains and issues including becoming entangled in the War of Kings; however, they face no greater threat than that of MCU background baddy, Thanos.
It's safe to say, based on some of the concept art and what we do know of the movie thus far, that we will likely see an origin story for Peter similar to the one detailed above. Just what the initial mission of the cinematic Guardians might be remains to be seen and there are multiple antagonists for them to battle along the way. We do know that Quill will be an arrogant and find himself in trouble. In my opinion moviegoers are going to really enjoy Chris Pratt's take on the character. Pratt is a goofy fella and this role will probably be his introduction to the masses.