Barbiturates. An OD on pentobarbitol would do it. You would fall unconcious and stop breathing. It's used to euthanize animals but is also now used for assisted suicide by those "right to die" organisations.
If that wasnt available just a massive overdose of strong sleeping tablets and some strong alcohol would likely work, but just to be sure I'd also try to asphyxiate myself at the same time somehow.
I dont trust drugs though, I'd prefer a shotgun.
Or better yet I just wont kill myself. I hope you dont either.