I am not sure about the earliest time that you can make reservations. You will need your resort reservation number. It will also ask for your last name, date of arrival, and resort in order to retrieve your reservation. If you haven't signed up with the WDW website do that too. You can put your reservations in your cart, continue searching, and once you have made all of the reservations you want, check out.
Just a word of warning, I found the online system to be very hit and miss. When it worked, it was great. If the restaurant I wanted didn't have the exact time available it would give me available times around the time I input. However, there were plenty of times that I tried to log in and a strange error code appeared. I just had to keep trying to log in until I was successful.