Once you have filled out the form, you will be shown a Preview of your listing. If the preview looks good, then go ahead and finish the process, or make changes and preview again. Your auction will start immediately, and run for the number of days that you specify. You must enter a minimum bid amount to start the auction. You should give some thought to this amount before listing.
Most sellers like to start the bidding a little lower than what they hope to get for the item.
Keep in mind that you are required to sell the item for the highest bid, with the only exception of a Reserve Price - details in the next question and answer. You may set a hidden Reserve Price higher than the minimum bid. This means that you are not required to sell the item unless the high bid meets or exceeds your reserve price amount. Your reserve price is not revealed to bidders - only that you have set a reserve, and whether it has been met. If the the high bid does meet or exceed your reserve, then you are obligated to sell for the high bid.