What strength e-liquid should I use?

This is a difficult one to answer as each vaper is different. A general rule of thumb is if you are a new starter smoking 20 a day you should start around the 18mg/ml (1.8%). If you start into sub-ohm vaping or variable voltage or wattage you might want to lower your nicotine strength. The best bet is to try a variety and see which suits you best.