I can't sleep well at night and I really don't know why. I want to have a better sleep every night. What should I do?

Consultation. Consult with someone trained in sleep medicine. There are many things that interfere with getting to sleep and maintaining good quality sleep. There are many do's and dont's to be aware of. Google 'sleep hygiene'' for some ideas and see a professional.
Sleep Hygiene. 1 dark cool room 2 no pets in room 3 no alcohol 4 no caffeine after 1 pm 5 no computer 2 hours before bed. 6 no tv 1 hour before bed 7 no exercise 4 hours prior 8 sex 30 minutes prior to sleep time (lots of it!) 9 yoga every morning.