Preparing Adventures
You might use a published adventure from Paizo or another company, or you might construct your own adventure as you prepare for your game sessions.
Published Adventures
A published adventure is prewritten, but it's not set in stone. Changing the details of an adventure to suit your group isn't just acceptable, it's encouraged! Use the backstories and predilections of the player characters to inform how you change the adventure. This can mean altering adversaries so they're linked to the player characters, changing the setting to a place some of the player characters are from, excising particular scenes if you know they won't appeal to your players, or including additional scenes and side quests tailored to the interests of your players or the background and preferences of their characters.
Starfinder Society
At the start of a session when you're running a Starfinder Society adventure (known as a scenario), you'll collect your players' information. At the end of the scenario, you'll write down the rewards their characters earn for completing the scenario on a chronicle sheet your players can keep for their records. The rewards they gain are detailed in each scenario. Once you report the session's results online, the rewards become a persistent part of these characters, even if they play in other games with other groups. These scenarios include important choices, and you can report what your group chose—decisions that will guide the future of the campaign!
Your Own Adventures
Adventure plotting can start at many different points. You might begin with a particular antagonist, then construct an adventure that fits that villain's theme and leads the group to them. Alternatively, you could start with an interesting location for exploration, then populate it with adversaries and challenges appropriate to the setting. You can start with even less—perhaps a moment or interaction you'd like to see occur, an item or subsystem you'd like to incorporate, a genre you want to explore, or simply an atmosphere you'd like to create.