After a longer cutscene you will take control of Desmond and will have to climb to the top of a building. Start off by using the opposite ladder and then follow the pipes to the platform with a fluorescent lamp.
From there jump left to the ventilation shaft and run onwards while jumping between the scaffolding elements.
Eventually you will reach an empty warehouse where you will have to slide below a half-open gate.
Outside, jump right and climb onto the red crane.
Further road leads around the building along the scaffolding elements.
After reaching a bigger platform, climb the wires to the upper floor.
On the right you should see a big spool. Slide underneath it and run onwards.
To get higher, climb the leaned fence and keep heading right.
Jump between the scaffolding elements to go round the building from the right side.
That way you will reach another crane from which you will have to jump.