Hope you have a fantastic first trip!
First, I would recommend to start getting a plan which park you might want to visit on which day. I know this does not yet have anything to do with dining, but you don't want to travel all over WDW to get to a restaurant. So, once you know, where you will spend the day, chose a restaurant in this park or at a resort close by. For finding out which days are best for each park, Tourguide Mike offers advice on this as well as the website of the Unofficial Guide to WDW.
Just as a side note, for a first time visitor, I would recommend to get as guide books the Passporter to WDW and possibly the Unofficial Guide. Both also have lots of info on restaurants, which should help you decide.
There are also a number of online resources with dining info, mainly allears.net (and overall very good and informative website) and here on wdwinfo.com at this link:
http://www.wdwinfo.com/wdwinfo/dining/diningmain.cfm With this you should be able to find places that sound good to you. The restaurant board here (which you already found, the first really good step to any Disney planning!!) is also a great resource for dining reviews. Just don't be too confused by it, there will always be people posting here who have absolutely different opinions about each restaurant.
As others have pointed out, 180 days in advance is the earliest you can make an ADR. I think, if you are staying on property, you can make ADRs for up to 10 days at 189 days from the first day of your stay.
The other question is, should you? That's difficult. A few restaurants sell out very quickly, especially it seems to be the case for Chef Mickey, the Princess meals, and Le Cellier. If you want to eat there, I would advise to book as early as you can. For the others, it still makes sense, to book ADRs as soon as you can. You still might be able to change a few things around later on, but I think it is always better to get an ADR than not to have one. If you decide that you don't need it, cancel it, someone else will be happy to get it!
Also, keep in mind, that September is the month where the "
Free Dining" used to be offered in the previous years. This is a special promotion by Disney where they offer a limited number of rooms for packages which include the Dining Plan without charging for it. This means that you get the credits of the (normal not deluxe) Dining Plan for free. This will not be applied automatically to an existing hotel reservation, you will need to make a new hotel reservation and cancel your old one in order to get this. There are a number of specialized internet travel agents (like
Dreams Unlimited which is a sponsor of this site, but there are others as well) who will monitor available discounts for you and change your reservation in order to get you the best price. I am using one for the third time and it worked great for me! For more info on discounts check out the code and rates forum.
Back to the topic of the dining plan. As this gives you a table service meal every day, during this promotion table service restaurants are in high demand and you will most likely not have any chance to find a restaurant that will take you without an ADR. So for September those ADRs are essential!
This has become rather long, but I hope it had at least some useful information for you!
And my personal recommendations would be Jiko, Boma, Kona Cafe and California Grill!
