Assassin's Creed 1 - HELP!?

OK so I started playing assassins creed number one a couple days ago because I really wanted to get into the series, but unfortunately I'm stuck, every time I go to a city it says that I need 3/6 side missions, then I will have enough information to assassinate my target, then it says to climb points to find those 3 side missions, but i have climbed all the climb points in the city, and I still don't know where to go to find the side missions so i can assassinate my target, and also I cant get to some of the districts of certain cities, it says that that information cannot be accessed at that time, please help!