Even though legally you’re an adult (or soon to be), most 18 year olds have a narrow view of the world. At 18, you are everything but fucked. Life is just starting. You may fail high school. The good news is you still have options. If you truly fail high school and don’t get a diploma, there’s the GED and then community college if that is what you want. If you don’t actually fail, but rather get horrible grades, the good news is that you can redeem yourself easily at a community college. Most community colleges will accept anyone and they’re very cheap. I’m talking about a community college, not a for-profit “college” where your credits don’t transfer anywhere. You go there for a year or two, get good grades and then transfer to a university.
But as long as you’re unmotivated, you are completely fucked I’m sorry to say. I’ve been there, so no judgment. You’ll find the motivation if you really want to eventually.