A lot of the corny adages are true. "One day at a time" is a good one. It sounds like you won't be cycling to a deadline so just concentrate on getting from A to whatever B you've picked for that day. You're young so you should get in shape pretty quickly. Start off without being too ambitious, say 20-30 miles a day. Before you know it you'll be up to 60-70. But remember it's a ride you're doing not a race. when you first set off you may well meet people going the same way. If they are faster riders than you let them go ahead. Don't spoil it for yourself (and others) busting a gut trying to keep up.
Stay at places recommended on the ACA maps but do check the addenda to make sure they are still in business. I learned that the hard way. Don't leave it too late before stopping for the day, if you aren't sure of your abilities start early and be done by 4pm. I can't recollect ever riding after dark in many thousands of miles of touring. Charlie, take a bike maintenance class NOW. There's not much that can go wrong that you can't fix yourself once you know your way round a bike. There's tremendous satisfaction to be had in overcoming the little challenges that get thrown your way: flats, worn out brake pads etc..
Tell your parents they can come and rescue you if it all goes to hell (which it won't). Before you know it you'll be 1000 miles away, having the time of your life and your parents will be bragging to the neighbors about how far you've rode.