Alcohol will not help,you might become dependant on it in the long run
Dont eat late at night,smoking dont help either,neither do tea or coffe,and dont go to bed with the tv on,
Bedrooms are for sleeping in,so iff your in your room all day,watching tv,on the net,playing games consoles this will have an impact.
Avoid sitting around all day,do something active,breaking a sweat every day will help you sleep,walking,cycling,going to the gym...
Maybe the reason you can't sleep is because you're stressed/anxiety from the GCSE's ?
I would say get out the house as much as possible,do something that takes your mind off it,even iff your in the shed lol
There are supplements available over the counter such as nytol one a night,but dont expect to take one and fall asleep in 10 mins
Iff it gets that bad,contact your GP and ask him/her for some Zopiclone,they'll knock you out in 20mins for 12hrs