Welcome!
When you start there is no easy answer for this. There seems to be a pretty broad range regarding how much nicotine people absorb from vaping so every individual has to sort this out for themself. For two people who have the same smoking profile, one person might get bowled over by 24 mg while the next person can find even 24 mg just isn't enough.
Ok, so what do you do? Try to find a place selling small sample sizes - like 5 ml bottles - and pick up one of each in 12 mg, 18 mg & 24 mg nic levels. If you do this none of it will go to waste. If you find 12 mg isn't enough you will just vape through it really fast. If 24 mg feels like too much nic you can just mix it with the 12 mg to cut the nic level down.
It's also a good idea to get at least one of those each in high or 100 PG and one in high or 100 VG, both to figure out your likes and dislikes there and to have something to change to in the rare case that you have adverse or allergic reaction to PG or VG. It's rare, but I found out right away I was allergic to PG liquids and at least I had a 100 VG liquid s right there to try.