Kudos to you for having the guts to try a thru! I hope you have a great experience!

I've never done one so I'm not as qualified to answer your question as some. But here is an idea that will get you started thinking about gear. Go to trailjournals.com and click on "Completed Thru-Hikes". This is SUPPOSED to list only the journals of those who have done a thru (of some trail, not just the AT) but for some reason it also lists some people who haven't started yet, etc. Just skip those and focus on completed AT thru-hikes. Now as you go down the list you will see that many people have posted the gear they used. Click on the links that say "View Gear". Pay particular attention to those who list lots of items. The people who only list 6 pieces of gear just weren't being very thorough. I guarantee they brought more than that. The most useful journals include a little review of whether they liked the gear or not.

This is a lot of fun to do. You can read some of their journals too to get an idea what a thru is like. Now every hiker does it his or her own way so you won't find a recipe here for a thru but after a while you will find some patterns emerging in gear. That will help you begin to form a list of things to check out.

In every category there are three or four most common items. For instance in shelter you will see a lot of tarp tents, a lot of MSR hubba tents, and a lot of hammocks. In rain gear you will see a lot of Marmot precip jackets and pants, a fair number of rain ponchos, and quite a few frogg toggs. In stoves you will find lots of homemade alcohol stoves, some MSR whisperlites, some jetboil, and a few wood stoves. You will end up picking one from each category that seems to make sense.

As far as going ultralight the most obvious thing it to buy lightweight stuff. But more importantly think about what you need and what you don't. Also you need to think of your gear as a system, not as a collection of individual items. You want as many items as possible to have multiple uses. For instance if you decide to bring trekking poles then it makes more sense to get a tarp tent or tarp shelter rather than a regular tent because then you can use your trekking poles to pitch your shelter rather than carrying tent poles.

I like going ultralight but that doesn't mean the same thing to everyone. We all go heavy in some area that is important to us. A thru lasts too long to be uncomfortable the whole time. A 3/4 ridgerest is lighter than a Big Agnes pad but that doesn't mean it will work for you on an AT shelter floor as well. Chemical water treatment is lighter than a filter but you're the one who has to drink the water. Go light on what you can and go heavy on what you need to stay sane.

Now be aware that you will probably change you mind on a few things with experience. Get a trial list of gear and try it on a week long hike. You may decide "hey this pad isn't soft enough for sleeping, maybe I'll try a hammock" or "this bag is overkill, I slept too warm" or "this chemical water purification tastes terrible I'll try a filter instead" or " this stove involves too much fuss and is unreliable". You final list may not look exactly like any list you've seen. But the mantra among hikers is "Hike Your Own Hike". Do it the way that makes you feel comfortable.

Best of luck to you.