Kinda the same here. My mind's constantly busy, and I get random thoughts popping into my head, that lead onto something else, that lead onto something else, and I usually end up being reminded of something scary so I get all stressed coz I'm on my own in the dark and have to spend ages thinking of something happy!
Try listening to music, ideally something relaxing. I usually take my I-Pod to bed with me and listen to it for a while - somehow listening to music makes me feel sleepy, whereas if I don't have my I-Pod on I'm lying awake not being able to nod off. I think it might be because when you're listening to music, your attentions are focused on that because of the aural stimulation, meaning your brain doesn't have to create its own stimuli. Or that could be nonsense, but anyway, I find it works for me!
Also you could try some aromatherapy oils - lavender is good, place a few drops on your pillow. A warm shower before bed and a cup of cocoa should help too!
I used to be unable to go to sleep without the tv on, or failing that, the light. A year or two ago I graduated to being able to have the tv on so quietly it was almost silent, but there was still that quiet buzz from it so I felt comfortable enough to drop off, and now I can have the tv off and sleep quite well.
When I'm stressing about something I've got to do the next day, or just general anxieties, I try to just tell myself to forget all about them and get to sleep. Let yourself feel that by getting to sleep you'll get away from all the problems, so that you won't lie awake dwelling on them. Also don't look at the clock to check the time - there's not much that makes it harder to get to sleep than thinking, 'Argh, it's 4am already, I'm only gonna get 3 hours' sleep!'