You will probably find destiny 2 confusing at first as there are plenty of modes, areas, and terms you need to get familiar with but like with the original, once you settle in they will become second nature. As you progress through the game the story missions will take you to four planets which will become unlocked for you to explore in Patrol mode when you reach a certain point in the game. In Patrol mode there will be Activities for you to find and complete, consumable items to collect, and enemies to shoot.
Activities that you can find include:
Adventures (mini story-missions)
Lost Sectors (mini dungeons)
Public Events (short objectives to complete with other players)
Each of the four planet's Patrol space will open up as you play the Adventures which are mini story-missions. You will be given maps to help guide you and there will be additional landing zones to save time travelling. If you are unsure on how to progress just go to the Destination screen and bring up the Story Milestones, each Milestone is comprised of a number of missions.