To make a trade, you must first post an order for your desired stock.
First, go to the list of available symbols to see what's trading on the exchange.
Using the order form, enter a price, a quantity, and click the appropriate button to buy, or sell.
The price you set indicates how much you believe the stock is worth/how much you think others will pay, and this may be different to the price you can currently buy/sell the stock for. (See "Limit Order" below).
Each trade on an exchange requires two people making matching orders.
The orders must also be at compatible prices, which means that the bid price must be greater than, or equal to the offer price.
The order book shows all of the orders that are currently active, but haven't yet been fully filled.
Orders are sorted so that the prices at the top ("top of book") are the best available.
The list also shows the number of shares available at each price level.
Limit orders are priced, and will not be filled at a price worse than the limit price (thus, a limit order to buy will never cost more than the limit price, and a limit order to sell will never make you less than the limit price).
Market orders are un-priced (any price entered will be ignored). The order will be filled at any price available on the opposite side of the book.
Time in force defines how long your order is allowed to remain on the book without being filled.
Good 'til cancel orders will remain on the order book until either they are filled, or they are cancelled.
Immediate or Cancel orders will be filled as much as possible at the time the order is placed, and are immediately cancelled.
Fill or Kill orders are similar to Immediate or Cancel orders, except they are all-or-nothing. If the entire order cannot be filled when the order is placed, it will not be filled at all, and will be cancelled.
When looking at the order book for a particular symbol, your position (the number of shares you have owned or borrowed) is shown beneath the order book.
You can also see a summary of all your positions by clicking the "Positions" link in the nav-bar.