I am not a doctor, and this is not medical advice. Follow it at your own risk.

Make sure you're going to bed at the same time every night (notice: that's every night. Not just weeknights.), and getting up at the same time every day (ditto), and make sure the delta between those two times is long enough. Not "I feel rested after 30 minutes of sleep" long enough: start with 9 hours and maintain that until you're getting up < 30 minutes before your alarm by yourself and feeling rested. At this point, you can more or less consider your sleep debt paid off. Then, back off the duration of sleep in ~90 minute intervals (that's the average REM cycle in humans) using the same criteria (note that average time to fall asleep is about 15 minutes). Most people won't get past a 7.5 hour sleep time.

Another thing that really helped me is cutting most processed sugar and all caffeine. As somebody who had a serious coffee and energy drink addiction, this was really hard to do, but it makes a big difference. If you "can't" cut caffeine, then try cutting it after 2pm or so. The biological half life of caffeine is 3-7 hours in adults, so when in doubt, go earlier than 2pm if you can. If you're a smoker, quit that too. Generally, stop putting stimulants into your body. Given that most of those stimulants also come with all kinds of sugars and other less-than-desirable additives, you'll be more healthy for it in the long run.

Along that same line, alcohol, despite being a depressant, is a good thing to avoid, especially in excess around bedtime. It might help you sleep, but long term dependence on alcohol puts you firmly in the "alcoholic" category, and even worse, it can cause or exacerbate insomnia.

When you're doing all of ^, you shouldn't need anything else unless there's some kind of disorder in play. I had https://en.wikipedia.org/wiki/Delayed_sleep_phase_disorder, which the article likens to "Social Jet Lag". It's correctable over time, though, in part by following the rules I put up there ^, and in part by following these next ones (these apply to other sleep phase disorders, including mild insomnia):

0) This is optional, but keeping a log of anything related to your body will provide you with useful data down the road should you need it. Treat this like a personal log file: timestamped entries for when/what you ate, what meds you took, when you laid down in bed, when your alarm goes off, when you get out of bed (those times are usually different for me :P ), your rating of your sleep on some consistent scale, and how long you exercise. The more data, the better.

1) Start with 6mg of melatonin 15 minutes before laying down in bed. Keep doing that for a week. If you are still having problems falling asleep:

2) 6mg of melatonin and 25mg diphenhydramine (Unisom). Note that this is an antihistamine, and that in some cases, they can react poorly with SSRIs (depression medication). I know somebody that ended up sleeping for about 48 hours straight as a result. They're otherwise pretty safe. After a week, if you're still having problems, and you're being honest with yourself about following the rules at the top of this post:

3) 6mg melatonin, 50mg diphenhydramine. This is about the most potent OTC sleep cocktail available. After a week and really, really making sure you're following the rules at the top - see how I keep coming back to that? They're important - if it's still not working:

4) Go to a doctor armed with the log from step 0 and get a prescription for Trazodone, which can (I'm told by my doctor) be safely layered on top of the Melatonin and Diphenhydramine. If you stay awake through this, there are other options too. Zolpidem didn't do anything for me, but I'm told it works wonders on other people.

In any case, start with the minimum effective dose of these things because they can really mess you up.