The strength (regular, light, ultra-light) of cigarettes you smoked plays a big role in what nicotine level to start with. If you smoked regular (full flavor), I think 18mg-24mg is probably a minimum. If you smoked lights, either 12mg or possibly 18mg. If, like me, you smoked ultra-lights.. well I won't say don't try 12mg, but be prepared for the possibility of nausea and stomach pains after you've been vaping for a few hours. If you do get that reaction, whatever level you choose, it's just a mild overdose of nicotine, and it will pass in a fairly short time, but you might want to consider dropping down to the next lower level, as I've had to do (18 and 12 both made me ill). I've found though, that with 6mg, I'm vaping pretty constantly, so I'm going to get some 12mg and combine with it the 6mg to make a 9mg for myself.
It's not necessary to quit suddenly, if you haven't already, although some people apparently find it very easy to do, once they start vaping. If you find the right e-juice, the one that hits you *just right*, it's possible that from that moment, you can drop the cigarettes like a hot potato. Or you might be more like me, and just find vaping so pleasurable that you find yourself smoking 1/4 of what you were used to, and not even missing it. That's how it's been for me, just a steady decline in the number of smokes per day, since starting *seriously* vaping, until now, at 1 a day, I'm ready to leave off the cigarettes entirely and just vape.
And no, it's not *exactly* like smoking; there are differences in how you inhale it, and vaping delivers nicotine somewhat more slowly than smoking does -- a few minutes rather than seconds -- but, it's so similar that it seems to scratch that smoking "itch." But like JaxMike said, there's no need to pressure yourself; once you find that perfect e-juice, you'll find the cigarettes just falling away. Seems a lot of folks here, with no intention to do, quit smoking "accidentally".
Andria