What do i need to conquer a city?

Right you have found yourself a nice little target that you would like to conquer?

Firstly there is a difference between conquer and enslave.

Conquer can only happen in a sub city (not a main), this is where you get to make it yours.
Whereas enslaving can only happen in someone else main city, ie there capital.

What do i need building wise for this?

You need to have a lvl 11 minimum palace, also the place you are attacking needs a lvl 10 minimum palace.

What position do i need?

The minmum position you need is senior prefect, you can check how many territories (enslaved capitals) you can have by clicking on your palace and hovering over the image.

Enslavement
Palace level Enslave # of capitals
0 - 10 None
11 - 12 One
13 - 14 Two
15 - 16 Three
17 - 18 Four
19 - 20 Five

Once you raise the city's dissesnt to 50 you are able to enslave it. Each occupy attack you send that is successful raises the dissent by 10, but if the person you are attacking is online at the time they can pacify to reduce the dissent.