I don't understand RAM. What RAM do I get?

[viprit]

First time builder here and all I have to do the shopping process is ... RAM ??? Why in the world is there no simple guide to them. They always get it to be too intricate.
Intel core i7 8700K on an MSI Z370 Tomahawk for a gaming / streaming / low-end video editing computer. What RAM do I get? Does CAS Latency matter? Does the jump from 2666 to 3000 matter?

https://tradevenue.se