Basic tutorial mission, so shouldn't be difficult. After the intro cinematics, follow the other guys into the cave. Desmond uses the Apple artifact to open a door to the Central Vault. Then slide down. Keep going through the tunnels. Desmond picks up the glowing cube and places it in the panel, activating some sort of machine (The Animus. Then he hears a female voice, Juno, in his head: "the key... you must... find...the key..."
Desmond says "Here we go again" before passing out. He wakes up in the Animus and is told that the Temple triggered a bleeding effect.
Software Update for Animus
The crew tells Desmond that they picked up a software update for the Animus and would like to run a couple of quick tests to make sure there aren't any major issues. This begins the tutorial. Follow the onscreen prompts and the objectives below to complete this mission.
Follow the path (Towards the Green Icon)
Climb to the marker (on Xbox Hold RT to start climbing and use L to move while climbing)
Freerun across the path (Xbox: Hold RT and move L to Freerun)
Perform an Air Assassination (Optional)
Kill the Soldiers(Targets Killed 0/2)
Jump over to objective