Each player can only have 26 cities so you need to choose you new cities location carefully. Cities with 6 farmlands might be useful at the beginning but you probably already realized that the quickly become obsolete since a 15c produces 150% more food than a 6c and there are plenty of farms to plunder for resources. 9c (9 farmland) are a good tradeoff between a 6c and 15c, they can hold a a medium sized army and have decent resources output, once they are built the way you want they can be use to feed resources to your 15c. 15 farmland cities (15c) are the best choice for supporting a large army. Make sure that you pick a spot where you can get 2 oases for it. It may be tempting to build cities close to your existing cities and/or outside the cluster in the middle of the map, but without oases they will not be very productive. When selecting the spot for your new city make sure that both it and the oases you want are not being farmed by other players. Since you most likely will be unable to take 2 oases right after building your city, make sure that you plunder often the cities with the oases to show that they are your farms.
Lobbyist vs. Pioneers
To build a new city, there is 2 options. Lobbyists will allow you to get a city with buildings already built but they are expensive and slow to use. Pioneers are cheap and easy to use, but you must build the city from scratch. Both methods require demolishers (unless you go with an unoccupied spot). For the Lobbyist you need to demolish the annex and be sure you are not trying to lobby the main.
One of the advantage to use pioneers for your new cities, is that since you rebuild the city from scratch your city will have the layout you are use too. If you prefer a certain buildings placement in your cities pioneers might be the way to go. With the lobbyist you will end up with your opponent layout which is probably a layout that you are not used so it might be annoying. Use the View Cities menu to manage your cities and it uses the construction date of the city to determine its place in the list, with the newest being at the bottom. when you lobby it, it might have been built long before your other cities and be placed anywhere in the list. Once the location is picked out, send your demos/rams to take out everything in the sub-city, destroying it and creating an empty space for your pioneers. If you do not have demos, ask your league leader for help.
Check out my previous article about lobbyist and pioneers :