When you were in kindergarten on the first day of class, your teacher asked you "What do you want to be when you grow up?" You answered things like, "Princess" or "Astronaut" or "President."
When you were in 5th grade you were asked the same question, and you said thinks like "I want to be a Doctor" or "Teacher", maybe even "a Singer/Actress" or "Police Officer."
Now, here you stand as someone who has graduated high school and is now entering the world, and they ask you this same question...only now, you have no idea. Suddenly, when they ask you this, it is no longer a hypothetical question. Now, they want a definitive answer, and when you don't have an answer for this question it can be a little overwhelming.
But I'm here to tell you, it is okay. It's okay to spend your first (and even your second) year of college with a major in "Undecided." It's okay to not know what you want to do every day for the rest of your life.
It's okay to think you want to be a doctor only to find out that the sight of bodily fluids makes you queasy (ME).
It's okay to go spend a year in the Teacher's Education Program before you realize that you don't want in the classroom all day.
It's okay to not know.
At 18 years old, the world is a scary and unknown place. Hell, I'm 22 and I can tell you the world still seems scary at times. At 18, you aren't even sure how to make Ramen without burning your hand, but suddenly you are supposed to know what you want to do with your life. Chances are you don't even know yourself yet! You probably don't know who you are outside of your small hometown, where you see the same people doing the same thing every day.
College is the time for you to make these decisions, it's a time for you to figure out who you are outside of being someone's child, or brother, or sister. So, explore your options, your interests, and your ambitions.
Maybe you do want to be president someday, explore that. Maybe you want to be a singer or a dancer, take a class as an elective and see if you are any good. Just don't beat yourself up about no knowing.