About an hour before you normally go to bed, start to "slow down". This means no activity, honestly. Be lazy, sit on the couch, and chillax.
Stay away from lights such as computer screens, cell phones, especially when your room lights are off. These actually disrupt the process where your body prepares itself for sleep.
If you need lights, use ambient lighting. This means use lets say, one lamp in your room that doesn't fully light every corner. I'll show you!
Like this! It's a dim effect, which relaxes you rather than that gross overhead lighting you see in classrooms. T'is a nono before bedtime!
Read. Draw. Write. The choice is yours. These are all activities that are calming and don't involve a lot of movement. Instead, they use your mind, which will cause you to become tired ;)
If you want to go extreme with ambient lighting, light candles! Those with scented wax are extremely comforting, this is personally my favourite.
BE FULL. Eating to the point where you are full always causes you to have a great sleep. Going to bed hungry is the worst.
Honestly try all of these things. They really help me to sleep :) oh and one more thing, don't go to bed angry, sad, anxious or excited. That keeps you awake ;)