In AC you will find that mission types repeat and you need to complete them in order to localize your target. Every task has always the same scheme - enter the city, synchronize on the highest buildings (you can find them on the map), get the information, go to the Bureau and after that find and eliminate the target. Before assassination you need to do a little investigation, find some clues which you find described in more detail below.
It's always the same. You need to find an agitator, usually speaking to a listening crowd. Target him and go after him. When he gets to an empty part of the street, punch him as long as it is needed to make him talk.
Sit down on the bench and find a pair of talking people. Target them and listen to what they're saying.
It's always the same. When your target leaves, go after him. When he stops and starts to look around move close to him and take what you need.
He will give you information in return for doing a little task. Usually you need to assassinate someone or find a set amount of flags in a given time
They're optional missions but can help you when you're running away or when you need to get past guards without calling undue attention.
The simplest and most common type of tasks in the game. Climb on a tower (it's marked on your map) and synchronize on the top - you will get missions in the surrounding area.