It sounds like you're trying to scan planets in a way similar to Mass Effect 2, but it works differently in 3. Instead of a mineral scanner, you have an area scan pulse that you activate from the map. If you happen to find something on a planet, you can then scan for that object specifically, but no planets are available for scanning until you get a ping from the map. If you get a scan ping out in open space, it's probably a shipwreck with fuel you can recover.
One last note: keep an red bar at the bottom-right corner of your screen carefully. Every time you scan, you risk alerting the Reapers.
