Planning your first trip is overwhelming. Just remember it won't all be perfect and things will go wrong, you need to "go with the flow". I start with time of year and double check my dates. I have so many newbies say to me - Oh we are going over Christmas, Easter, 4th of July. Then come back - I was so crowded, we hated it. DUH! From there I looked up park hours, made a list of must see attractions. Tried to figure out if I was doing early entry or not and scheduled a few PS ressies for each day.
Another easy way is to book through Dreams Unlimited on this site. They do wonderful at getting the lowest room rate and making all your PS ressies. Plus they give you a complete itinerary. They are super at getting the hard to book places like Cinderella Castle breakfast. I used Cathy Miller, she was the best. I send all my newbie friends to her, all of them have been extremely happy. My 4th friend is booking a trip through her.