Well, here's what I know.... definitely not everything, because I'm in the middle of planning our first trip since FastPass+ began. First step is to create an account on My Disney Experience. Then link your park tickets, your resort reservation (if staying onsight) your dining reservations, etc. My Disney Experience will tell you on the homepage (once you sign in) how many days until you arrive at your resort. I can't speak to hotels not on property, but I think you can make FastPass+ reservations 30 days in advance if you are offsite. 60 days if you're staying onsite.
So... with your dining reservations made, you should have a pretty good idea of which park you'll be in each day of your stay. Think about what time of day you want to experience popular attractions. When your 60 (or 30) day mark hits, you can reserve your FastPass+ times from the My Disney Experience website beginning at midnight eastern time of the day you go from 61 days to 60 days. That's the basics of how to get started. There is a wonderful thread on these boards that I'm sure someone will link for you, by Mesaboy, or just scroll down the list of threads. It will most likely be in the first two or three pages, as it's very popular. It will get into the nitty gritty about which rides and attractions are most urgent for getting a FastPass+ for. You can pick three per day, but all three must be in the same theme park (Magic Kingdom, Epcot, etc.) After those three attractions have been visited, you can visit the kiosks in the park (like the old fastpass system) and get additional fastpasses, one at a time.
You are not locked into your Fastpasses - you can change them (it seems I've changed them every day) - as you finalize plans. Some attractions will be more difficult to get than others, but you can switch them around pretty easily. It just takes some getting used to the website / app. You can do it on your smart phone or your computer.
I hope that helps you get started at least. Have a fabulous trip!
