Just go around the system, scanning. If your scanning pulse detects something of interest, it will appear as an icon on the Galaxy Map. Simply go over to it and access it. If it's a planet, you'll have to find the asset in a manner akin to ME2's planet scanning; if it's a wreck in space, then you only need to access it to get the resource or asset.
Each scanning pulse you use in a system controlled by the Reapers will raise the Reaper awareness meter. Once the meter is full, the Reapers outside of the system will come in and attempt to capture the Normandy; you'll have to flee the system in order to escape the Reapers. Reaper awareness resets after completing a mission. --- Lightning. It flashes bright, then fades away. It can't protect. It only destroys.