You might try taking some melatonin, 1mg to 2mg about an hour before you want to sleep. Melatonin is a hormone that your brain releases in response to darkness, so melatonin is a hormone your body already produces. Melatonin is available over the counter, and does not require a prescription. The main side effect is sedation- and that is the effect you want if you want to get good sleep! The best thing about melatonin is that it is natural, and is something your body already produces.
But melatonin will not 'knock you out,' meaning it is not highly sedating, so you still need to practice good sleep hygiene. Good sleep hygiene includes: going to sleep at about the same time each night; waking up at about the same time each morning; avoiding caffeine after 6pm; engaging in relaxing activities in the evening, like reading; and getting good aerobic exercise during the day (so that you have a good reason to sleep- you're tired from the workout). Also, do not do anything in your bed besides sleep. Your bed should be for sleep only- do not do work on your bed, don't watch TV in bed. We are creatures of habit, so if you use your bed only for sleep, then you will get in the habit of sleeping when you are in bed. If you are tossing and turning at night, get out of bed, and do something else for a little while. If you just stay in your bed and are tossing and turning all night, then you will just be more frustrated and this leads to more insomnia. So when you do something else for a while (nothing strenuous), then return to bed, and just allow your mind and body to relax.
If you continue to have sleep problems despite these recommendations, then you should visit your doctor for a full assessment, as there may be some medical problem that is interfering with your sleep. If you are indeed stressed-out, you may also benefit from seeing a therapist.
