Most common reasons for not being able to sleep are lack of physical activity during the day, eating too soon before bed, too much caffeine, your mind being occupied & poor sleep patterns by staying up half the night and then trying to go to bed at a normal time the next day.
I've found when I'm getting regular physical exercise I can't help but go to sleep. It is all I can do to keep my eyes open! If you slob around all day and take a bus or drive everywhere then your body isn't exerting itself at all. Physical activity will help tire you out and give you endorphins so you will feel better when you are awake. I challenge you to not sleep if you've done a lot of exercise..it's next to impossible.
Secondly don't eat right before bed or your body is kicking into digesting mode while you are trying to sleep. Not only that but you'll get all the energy from your food arrive when you are at your most inert.
Limit the amount of tea, coffee, chocolate and other items containing caffeine you consume during the later afternoon and evening. If you are filled to the brim with stimulants no wonder you are awake!
If your mind is churning then you need to switch off before trying to sleep. As someone suggested a hot bath or shower is good for relaxing. I fail to believe that there is never any hot water in your flat, especially in the evenings as most people I've lived with have a shower in the morning anyway. Switch your hot water heater on and you'll get your hot water 20 mins later. Do a light activity which will make you sleepy..not internet or tv. Reading a book, listening to quiet music and laying on your bed in your nightclothes whilst you do it. You need to tell your body it's sleep time.
Last and most importantly is get into a regular sleep pattern. Most people will find it really hard to sleep if they are trying to sleep at 10 because they have to be up in the morning...but for the past 2 days they have been up til 3. When I'm in a regular pattern I'll go to bed at 1 and get up at 9 like clockwork. My eyes are so heavy by 1.10 that I can't stay awake and my body clock wakes me up on the dot of 9. Obviously it's not always possible to follow your usual pattern, but still getting up at the same sort of time on weekends and stuff will help you. Set an alarm for 9.30 on the weekend, despite what time you went to bed and the following night you'll be really tired by the usual bedtime.