When you're done with all 4 chapters, look up a build guide for the armor and start putting together the rest of the supporting pieces in the way the guide describes. As you work your way through the chapters you'll be given pieces of armor as a reward for chapters 2,3,and 4. Once you're at 70, start following the chapters of the seasonal journey (these are not acts in the campaign). Use the leveling guide to get to 70 (since you've never played you will be doing the 'no challenge rift cache' variant). After you complete (or skip) the story then go into adventure mode.

When you make your first character, make it a seasonal one. In short: ignore the campaign/story unless you want to experience the story, in which case go ahead and do that. Here is a complete beginner's guide for Diablo 3 that will get you started.

Otherwise ignore everything I typed below, unless you have the RoS expansion and can fully update it (some console versions cannot). If you want to have more fun, get the expansion. EDIT - since you only have the base version, you can only play Acts I-IV of the campaign.

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