The only ways to get YoCash is either to purchase it with real money or Facebook Credits (these can be bought using Facebook and are tied to your account), or, to earn them through filling in surveys prompted to you in game.
Keep an eye out for promotions that reward you with YoCash. Any other methods for getting YoCash that are not through official channels are likely scams and could lead to your account being banned or suspended.
This is done by clicking on your inventory which is the button at the bottom of the game that looks like a box with things inside. Click on an item and then click the 'Sell' button. As in real life used things are not worth as much when they are new but you can get some coins back.