The price you set for limit orders depends on how quickly you want the order to fill and on your opinion about where price will be in the future.
If you are willing to wait and think that price will move in your favor in the future, then just set the limit buy or sell price to where you think price will move in the future.
But if you want the limit order to fill as quickly as possible, look at the current order book to determine your limit price. You can find the order book under the "Order Book" tab at the bottom of your order form. For example, suppose you want to buy 20 bitcoins. Look at the "Selling" side of the order book (see picture below). This shows you the best (lowest) sell offers that are currently on the book. Look at the cumulative volume to see where you need to set your limit price. Looking at the picture below, you can see that you would need to set your limit buy price to $297 or higher to immediately buy 20 bitcoins. Assuming the sell orders don't change, setting your limit buy order at $297 will be just high enough to buy 20 bitcoins from the lowest sell offers.
If you want to sell bitcoins instead, the process is similar, except you would look at the cumulative volume on the "Buying" side of the oder book, which shows the best (highest) orders to buy on the order book.