Play AC2 and Brotherhood before this, AC1 is boring and sucks other then the cut scenes in Abstergo between memory sequences, watch those online and save yourself 20 hours or so, Revelations can be skipped, it did nothing to advance the gameplay. AC2, Brotherhood and AC3 are very good however.
There are 2 parts the story in the Assassins Creed series: The modern story(Connects over the entire series), and the historical story.(Changes with each new assassin)
There is a short summary at the beginning of AC3 that will recap most of whats happened in the Modern story.
If you would like to see a more detailed summary of what is going on in the modern story or read a detailed summary to see what has happened in the lives of the previous assassins Altair and Ezio then you can go check the Assassins Creed Wiki, im sure it has more than enough information to understand what is going on in this series.