If you watched the Hobbit and wondered why all those wizard meetings, Necromancer?, do we really need a trilogy" and so forth then you might want to read a nice summary of the important moments in history of Middle-Earth that leads to the events of The Hobbit and why exactly Peter Jackson focused on the moments he did. As I said in my review, the movie was long but short of cutting action scenes, I had no clue how he could have made the movie shorter because those scenes were needed for the larger tapestry that he is putting together. Click here and you can get an idea on why I came to that conclusion. The article also serves as a nice takedown of critics who attacked the film for the wrong reasons. Below is aquick summary of the same with more added from LOTRProject's and Tolkien Gateway timelines. Spoiler warnings for article and below.
1999 - Erebor founded, Arkenstone discovered.
2460 - Sauron returns with Dol Guldur as his base.
2770 - Smaug the Dragon conquers Erebor taking over The Lonely Mountains. The War of Dwarves and Orcs starts. Meanwhile Sauron (aka The Necromancer) gets one of the dwarf rings of power from Thorin's father.
2850 - Gandalf learns of the Necromancer at Dol Gulder. This is changed into the the film to be Radagast the Brown during The Hobbit events.
2851 - Saruman the White (aka bad guy who captured Gandalf in LOTR) decides he wants to control Middle-Earth by getting the One Ring. Would not be surprised if the Hobbit trilogy shows his change in ambition at some point.
2861 - Orcs invade Rohan, killing its king.
2885 - Sauron's allies invade Gondor. Rohan provides support to stop the siege.
2933 - The child Aragorn, King of Gondor, hidden and raised by the Rivendell elves.
2939 - Sauron aggressively begins looking for The One Ring, having no idea that Gollum had it. Nor that Bilbo recovered it.
2941 - The events in The Hobbit
2942 - Bilbo returns to Bad End and Sauron returns to Mordor
2951 - Sauron reveals himself to Middle-Earth, rebuilds Barad-dur (the other tower in LOTR The Two Towers)
3000 - Saruman allies himself with Sauron
3001 - During Bilbo's birthday, Gandalf discovers he might have The One Ring (The Fellowship of the Ring)
3001-3016 - Gandalf attempts to determine how to identify The One Ring
3017 - Gollum captured by Sauron's forces, Gandalf learns how to identify the ring from scroll in Minas Tirith
3018 - Gollum confesses "Baggins" and "Shire" as location of ring, later escapes to hide in Moria.
3019 - The rest of the events in The Lord of the Rings trilogy
3021 - The Three Keepers (Elrond, Galadriel, Gandalf) and two Ring Bearers (Bilbo, Frodo) head into the west ending the Third Age of Middle-Earth.