A long day on the bike should include stops for proper meals at proper mealtimes - ie a decent lunch break.
Make sure you have enough liquid to keep you going to the next break - don't set off without two full bidons, and use electrolyte tabs (they will help keep your digestive system in order). My strategy for hydration is to make sure I take a swig every 10km - it's good to have a plan otherwise it can be easy to forget to drink often enough, especially if it's not a warm day.
Keep some snacking food in your jersey pockets for between stops - use energy gels if you like, but fruit is also good (eg banana, apple chunks in a sandwich bag, raisins, dried apricots), jelly babies, flapjack etc. Savoury snacks are also good - bits of cheese wrapped in chorizo are a favourite, or hard boiled eggs.
But don't overdo the snacks, just keep riding at a steady pace so you don't burn out too quickly, and top up your energy levels occasionally.
Make sure your bike is in good order before you set off, but also make sure you have everything you need to carry out roadside repairs - ie puncture kit/spare tubes, spare chain link, multitool. And practise using them at home if you're not confident you'll be able to fix things at the roadside.
Also make sure you carry a rain jacket just in case.
Don't forget the sunblock either - even if it's not a sunny day, being outdoors for 12 hours will expose you to a large dose of UV.
Plan your route carefully. Use Google Street View to check out any potentially tricky bits you're not familiar with.
Above all else, just get out and enjoy it. All of the above advice is designed to make life easier for you, but you'll get by even if you don't follow it - when I was 15, my best mate and I decided to ride from Ramsgate to Brighton one weekend (about 100 miles). We didn't plan it at all, or prepare in any way, just got on our bikes and rode. And it was great!