How to trade .. who do I trade with?

To trade with other players (in your alliance) you need to be a trader class. You can then tap on their city in the world or find them in the alliance member list and choose the 'trade' option. You can then send them your resources. You can also use the Market Building in your city to buy items.

Trading is now also part of the Commercial hub (helicopter pad) and anyone can trade here with different items appearing each day.