First you need to buy a Parachute from the tailor, if you dont have one. Then climb the Galata Tower (which is a few steps from the Assassins HQ, in north of the City.) Then Jump and Press X to open the Parachute and glide in to the golden horn (into the water)
FinnegansDirge might add not to jump off the side with the haybale below it... a few of my first attempts ended in a Leap of Faith instead of parachuting :/