I know it seems completely undoable, and when the idea crossed your mind you probably shut it down almost immediately saying "I'm in high school, and broke theres no way I could afford a trip like this!" Trust me when I say you can. I did it (with the help of graduation money from family) with one of my best friends, and it's because we planned literally everything.
1. Plan Everything
One of the reasons our road trip was so successful is because we started planning it about a month out. We figured out the routes we were going to take, where we're going to camp, and the places we were going to stop. Because of this we had a rough estimate of how much it was going to cost us, which helped us figure out how much money we had to spend in other places.
2. Consider Camping
When my best friend and I did it we camped the entire way, which saved us a TON of money. Renting a site only normally costs around $20, and compared to anywhere else you might stay that's really cheap which makes this a great option if you have a tent laying around.
3. Buy Food Rather Than Eating Out
Another MAJOR money savor, we bought food before the trip. Things like Cereal and bagels for breakfast. Sandwiches and chips for lunch. A few random things for dinner, and of course some snacks for the road. We bought all of it when we left, and just kept the things that needed to be cold in a cooler and bought a bag of ice everyday to fill it up. Not only did this save money, but it also saved a lot of time when we were on the road because we didn't need to pull over and wait to get food.
4. Pack Lightly
Try to keep everything down to one bag, especially if you're camping. We had a tub for our food, 2 bags, our tent, and a few other items all packed into my little Honda Civic which was a little tight. Not to mention packing lightly also helps on gas mileage so your car doesn't have a as heavy as a load.