N400
 

I was brought here under legal guardian ship when i was thirteen years old, I was adopted by a U.S citizen. I became a legal permananet resident when I was 18, now I am trying to apply for my citizenship and I'm not sure which form I should fill out the N-400 or the N-600? I wonder about the form because of the recent application for my U.S. passport was denied, I was trying to apply for the U.S passport under the "parent is a citizen".

N-400 seems to be the right form for you, usually you have to meet certain requirements to acquire citizenship by birth or after birth but those requirements need to be fulfilled before 18 years old. You can look it up here https://usimmigrationforms.com