The way to pay money:
Select the player you want to trade with, click the mouse's right key on his portrait and select "Trade".  After he agreed to trade with you, you input the money amount at your trading column.  After the two sides both clicked "Lock", then click "Trade" to complete the trade.

The related information will be shown in the chatting box after the trade.

Is there any other place for me to check my items if my item box is full?
You can find the Warehouse Man in some towns, and you can check your items there.

How do I store and fetch my items?
Select the Warehouse Man and click "Warehouse".The interface for warehouse will appear.      Draw the items from your item box into the warehouse interface to store it.  Draw the item into your own item box if you want to fetch it out.

How do I deposit and fetch out the money?
Select the Warehouse Man and click "Warehouse" and "Deposit".The interface for deposit money will appear.      Input the amount of the money in it and click "Ok".  Click "Withdraw" if you want to fetch out the money.  Input the amount in it and click "Ok".