Hi!
You made the most important decision already which is to start planning. The rest can be done in steps.
With two young girls, I recommend visiting the Magic Kingdom and Disney's Hollywood Studios. Both of those theme parks are great for young children. I recommend you set aside one full day for each and if you have the time and money, spend two days full days in the Magic Kingdom; your daughters will love it! If you still have more days left in your vacation, then visit Epcot and/or Disney's Animal Kingdom. Since this is your first time visiting, I would not bother with getting the Park Hopper option.
With regards to dining, I think the first step is to determine if you're going to want to eat in full service restaurants. If you do, then I suggest making dining reservations once you know when you are going and have some idea in which park you will be on a given day. Since your daughters love princesses, I highly recommend that you consider one of your meals to be the
Once Upon a Time Breakfast in Cinderella Castle at the Magic Kingdom or the
Princess Storybook Dining at Akershus Royal Banquet Hall in Epcot, assuming you're going to Epcot. It's expensive but a lot of fun, especially with young girls! Both of those are very popular and dining reservations can be made up to 180 days in advance; I know that sounds crazy but they fill up and my own experience is that if one wants to eat in Cinderella Castle, one really needs to make the reservation at the 180 day mark.
You can make a decision on whether to get the Disney Dining Plan at a later date. Concentrate first on when you will be visiting and what theme parks will you be in, make dining reservations for the full service restaurants you are interested in, and select a hotel!