You can trade cards using different platforms like the Facebook Messenger. Simply talk to other players/friends and start trading.
In order to send a card you have to open your card collection in the menu, press on the set, which includes your card and hit the “send” button on the specific card. Next you can choose which friend you want to send the card to. Keep in mind you can only send the specific card, if you got more than one of it. Gold cards can only be traded during special events!