The panel where you can place orders is by default on the outer left of your dashboard. Select Market to continue.
Fill out the Quantity field. This is how much you want to buy or sell. Placing a market order means you agree to buy or sell at market price, so there is no price choosing here.
You can check the price on top of the dashboard, or in the Orderbook on the right. The lowest price in red is the best market ask (how much you will spend to buy 1 XRP for this example, or how much people are asking for 1 XRP). The highest price in green is the best market bid (how much you can get from selling 1 XRP, or how much people are bidding for 1 XRP). Market orders are executed with these prices. The difference between them (0.00005 BTC in the screenshot) is called spread. The bigger the spread, the more volatile the market.
Now let's say we place a buy market order for 1 XRP. Put 1 in Quantity field and click Buy. Dashboard will ask you to confirm your order, so do give it a check before clicking Submit Order.
Average price here is the best price right before the order is submitted. Actual price when your order is matched can be a different one - see Slippage. Fee for a market taker trade is 0.15%. Here it is being displayed as zero because there are too many zero in the decimal part, so hover your cursor over the number to see the actual fee.
This is the system confirmation that you have placed a market order successfully for 1 XRP at 0.00008 BTC. Click Dismiss to close it.
(open the image in a new tab to see it in full size)
Now you can see that your balance has changed, to 1 XRP and 0.01591 BTC (top left). On the right of the dashboard, the order we just placed is reflected in the Orders block, with a "filled" status. Its execution is right below in the Executions block.
Note that in all orders, slippage can happen.