You trade/sell items in the Exchange (
Here )
You could request that whoever buys the items from you starts the trade so you can see how trading works, but in general you would hit the "trade" button, then insert the username of the person buying your item. hit submit, then it will take you to a page with all of your items in little boxes. Click the items that they purchased and the click the start trade button which is found at the bottom of your inventory page. Do NOT hit confirm on the trade until they have entered the gold/items you are trading for. When they have entered the payment, and confirmed the trade, you will confirm and the trade will be completed.
Yes, even though you just joined, you are welcome to use the exchange. I used CI's as a bit of a gold booster when I first joined too ;D