Hello and welcome, Jeancairns.
I believe you've gotten the answers here about percentage of nic in the liquid, vs. how many milligrams per milliliter. Buying pre-made liquids, at least at first, will mitigate against making a mistake that could be harmful to you.
I started at 18 mg/ml while trying vaping but still smoking. A few weeks after quitting cigarettes completely, 18 mg was too much, and made me feel wired, so I dropped my nic to 12 mg/ml, the level I've kept since 2012.
I wish you the best vaping experience.