Come so far, but got so far to go

So school is coming to an end for me and even though I should have some idea of what I want to do with my future I seriously have NO IDEA!!! I know I am supposed to have some idea of what I want to do with my future, but in all honesty there is not a single idea in my head.
I have always wanted to go to into enviormental studies/marine biology/oceanology, and I still really do want to pursue something along those lines. But I don't have the best marks in math or science. Then I thought I wanted to enter social work, but I keep getting told I would be really good at law. So that just opens a brand new can of worms that I have no idea what to do with. I want to have a job that I love and have a passion for, but I also want to make sure I have a steady career and something that will last for a long time and doesn't have a risk of putting me out on the street.
Why does deciding my future have to be such a daunting task? I am only 18 years old. Deciding my future for the next 10 years shouldn't be something I have to do. This is supposed to be the time to make mistakes and learn from them. To fall, fail, and falter. To take a course on Lady GaGa cause it seems fun, not because it has any purpose of furthering your education of getting you a better career. I just wish everything could be like when I was 5 years old and the only thing I had to worry about was what was for dinner that night.

It seems that once you pass that new half-way point, a new one appears, but it seems so much further away than the last one.
I've come so far, but I still have so far to go.