The first phase of the MCU takes place between 2008 - 2012 and comprises six movies in total. Although the movies were released out of order to the 'universe' timeline, the phases are marked by release date. If you'd like to watch the movies in chronological order, please view this timeline. It also includes the shorts and the TV series.
Phase One kicks off with the introduction of the main six Avengers. It focuses mainly on Iron Man's story, as well as the formation of the Avengers. Note: Although the Hulk is played by Edward Norton in 'The Incredible Hulk', the actor is later changed to Mark Ruffalo, the current Hulk.
"After being held captive in an Afghan cave, billionaire engineer Tony Stark creates a unique weaponized suit of armor to fight evil." (IMDb)
"Bruce Banner, a scientist on the run from the U.S. Government, must find a cure for the monster he turns into, whenever he loses his temper." (IMDb)
"With the world now aware of his identity as Iron Man, Tony Stark must contend with both his declining health and a vengeful mad man with ties to his father's legacy." (IMDb)
"The powerful, but arrogant god Thor, is cast out of Asgard to live amongst humans in Midgard (Earth), where he soon becomes one of their finest defenders." (IMDb)
"Steve Rogers, a rejected military soldier transforms into Captain America after taking a dose of a "Super-Soldier serum". But being Captain America comes at a price as he attempts to take down a war monger and a terrorist organization." (IMDb)
"Earth's mightiest heroes must come together and learn to fight as a team if they are going to stop the mischievous Loki and his alien army from enslaving humanity." (IMDb)