I, like kitsune, started @18mg strength.
IMO, its a good starting level. The longer you vape, the more you'll start to find the level thats right for you.
As for what's equal to what - thats too much trouble. I bought my first e-cig based on the manufacturer saying that a cartridge equaled X cigarettes or 2 carts equaled a pack. Needless to say, I was quickly disappointed.
In the beginning I used my e-cig like I smoked analogs. Used it only on breaks or when I stepped outside. I also tried to "count puffs" to make it equal to the amount of drags I took on an analog.
This made it pretty easy for me to find if my e-juice was giving me the "fix". If I still had craving's, I tried a higher mg juice, or if I got a quick head rush or a nicotine headache - I'd back the mg's off.
Nowadays - I use 24mg when I really have craving's or want a stronger throat hit. My "all day" juice is typically 16-18mg. At night, I back down to 6mg or just use a 0mg liquid.
Hope you find what works best for you.
If you have more questions, don't hesitate to ask! Also, Welcome to VaperTalk.
Happy Vapin'