I think disney planning is a little overwhelming too! But then again, I'm pretty Type A and like to know as much as I can about anywhere I'm traveling. I don't like to plan every moment, but I do like to have full info so that I can make good decisions. We just came back from disney yesterday, and I started planning our trip in August. So if I can do it in only 4 months, you have plenty of time!
As others have said, start by thinking about how much you want to spend, and start exploring your options. Mouse Savers is great for info about what, if any, discounts you're likely to see when you're thinking of going, which might help you narrow things down. Look at the crowd calendars (Easy WDW has a nice free one, or the Touring Plans one is excellent if you want to subscribe, which I found very worthwhile but YMMV). Consider how busy the time is when you're making plans. For us, I learned from all the crowd calendars that the first week of Jan was only moderately busy, so I took the risk and didn't book a hotel until discounts came out (Oct) and I could make a final decision. We decided to stay on site for a variety of reasons, though I did explore some off site options too and they looked great.
One thing I DID do as soon as we had decided on dates, though, was book restaurant reservations. Unfortunately 4 months out I was already too late for some restaurants, namely dinner at Be Our Guest and Cinderella's Royal Table. Otherwise, I was able to get times at all the other restaurants we wanted. If you want one of the most popular restaurants or you're going during a really busy time, I'd suggest locking in your ADRs at 180 days. You can make changes later, but don't necessarily worry about planning all your hotel details before thinking about restaurants. I guess basically if you have any of the very popular stuff on your must-do list, be sure to book those things as soon as you're able, whether that is an ADR, FP+, etc. The lower priority stuff can wait a bit.
As someone else suggested, books are a great way to learn the basics. My personal fav is the Unofficial Guide because it appeals to my data-loving nature, but there are lots of other good choices out there. See what your library has (but I'd suggest looking only at the 2014 or newer titles because so much has changed), or buy some ones that look good to you. Once you have the basics, do more exploring on sites like Mouse Savers, Easy WDW, etc. And finally, these forums are great once you want to investigate specific topics or learn more about what people are saying.
Have fun!