I'm doing some maths revision, also kinda weak in Number Theory. I would appreciate if I could get step by step explanation for the following question:

Find a primitive root for Z^{*}_{19. }Z is integer. Attaching the image since I can't add the Z symbol

The answer is 2,3,10,13,14,15

I looked at some video's on primitive root, correct me if i'm wrong here, First I need to find the Totient, which is 18 since 19 is a prime (n-1), so how do I get 2,3,10,13,14,15 ?

Note to moderator: I tried posting this question earlier, for some reason I cant see my post. Kindly delete this post if its duplicate.