When i go to bed, a particular song keeps coming in my mind, and as a result, i cant fall asleep in time. what should i do to 'shut down' my brain ?

Insomnia: can be difficult to treat. You are likely not ready to go to sleep ( my guess is you force yourself to bed early so you can wake up early for work or school ) One way to combat this is to " pull an all nighter" on the weekend. The next day you'll run out of steam and get sleepy early. If the music persists try a white noise generator at night.