18 years old and have horizontal lines under my eyes.

It is very difficult to ascertain what the underlying problem and treatment in the absence of any eyelids photos. Nonetheless, what you describe may be due to malar creases and can sometimes be improved with injectable fillers. If the lines are due to lower eyelid fat bags, you may be a candidate for a lower lid blepharoplasty. If the lines are the result of hyperactive eyelid muscles, the Botox may be an option. It would really help if you could send some eyelids photos.

It is good to remember that when you smile, it is natural to make lines in the lower eyelid. These lines tell people that your smile is genuine. No lower eyelid/crows feet lines with a smile suggests to people that the smile is disingenuous. Generally fillers can be used to soften these lines. At 18 you are probably not quite ready for these types of treatments.