Is it only a sleeping issue? You are 18 years of age, are you having any stress like studies, exams or result stress?
It seems likely that your sleeping problem may be associated with some life stress. In that case, counseling will help.
If you have no other issues, try these simple tricks.
Try to keep away from TV, electronic gadgets like mobile, video games etc. at least 2 hours before bedtime.
Maintain a bed time schedule, including sleeping at a similar time everyday and have a warm shower before bed.
Take warm milk half a cup before bedtime, it helps release tryptophan, which helps sleep better.
Workout or exercise in the day .
Rid yourself of negative thoughts and practice breathing exercises to calm yourself.
Check your blood sugar if you get up frequently for passing urine.
Try the above or consult a counselor or doctor if the problem persists.
All the best!