Taking into consideration the strength and amount you smoked, and the type of hardware you'll be using to vape, is the best place to start, as indicated above; BKTOAD's suggestions seem very sound. But also, every individual has a different tolerance for "just plain nicotine" -- without all the other chemicals in cigarette smoke, some of which are there solely to make smoking less irritating yet more addictive. I smoked for 39 years, at some points smoking as much as 3.5 pks a day, but was down to about 3/4 of a pk at the time i switched to vaping, AND, I smoked ultra-lights for about the last 20 yrs -- so, even though I was using a puny little cigalike when I started vaping, I couldn't use anything higher than 6mg at first, without terrible nausea; when the cigarettes were completely gone, I went to 8mg, 9mg, and finally 10mg, and parked there for some months.
I personally think it's better to start low, and go up a bit if you find that you need to, rather than starting high, finding yourself feeling sickened and maybe less thrilled about vaping. Don't give yourself a reason to dislike vaping.
Andria