The Tutorial Missions are the best place to start and can be completed at your own pace.
You’ll be prompted to go through the tutorial when you first enter the launcher, which will lead you to have a closer insight into the game and guidelines on how to best get started.
You can also choose to “skip it for now” if you are already familiar with how to play Gods Unchained before, but please bear in mind that you will receive three core packs upon completion of the tutorial.