Do a Bit of Planning
For many people the whole point of a road trip is that it's an adventure, with no planned route and no strict time limits. You will encounter surprises and mishaps along the way, but that's all the fun of it. This is pretty ideal, however heading off on your big road trip without having done a scrap of research can land you in some pretty troubling situations. For example, if you don't know much about cars and are either taking your own car or plan on buying a cheap old vehicle, then it's going to be pretty frustrating when you have mechanical problems that may be pretty simple yet end up ruining your trip. It's important to know the basics of how a car works, so that you know how to replace oil, or recognise a simple problem if it arises. You should also take the car for a quick check-up at the mechanic before heading off, and be sure to know the phone number for roadside assistance should you need it.
Secondly, even if you don't want to plan your trip down to every minute, you should at least have a bit of knowledge about where you're heading to, what the roads will be like, where you'll be able to buy food and petrol and where you will be able to camp or find accommodation.