It really depends first on how you like to consume your cannabis? Do you like to smoke, vaporize take edibles or tinctures?
Once you have that decided, you should choose a pure indica variety, of a strength appropriate to your experience and tolerance. Go for an indica not a sativa as that is what will make you sleep.
There are lots of choices in indica tinctures, some of them formulated specifically for sleeping. I like the Alta California Botanicals – they have one called called Insomnia:
The other thing to consider is that if you smoke, the effect is going to come on immediately, but then will likely wear off within 4 hours or so, at which time you may wake up. If your problem is staying asleep more than falling asleep, you might wan to try and edible as the body metabolizes this over a longer period, and so you might find you stay asleep longer.
Try a low does indica, about two hours before bed. Start with 10mg and work you way up from there.
Hope this helps.