If a planet has not been scanned by you and is discover-able, it will have the tag "Unexplored" on it in your reticle. If a planet is already named in your target reticle then you cannot scan it. If the planet shows no information at all in the reticle then you need to run another pass with your Discovery Scanner.
The Basic Discovery Scanner has limited range. So if you do a scan at the main star in a system when you jump in, it will only find celestial bodies within a limited range of that point. So if you fly further out from the star, you will probably need to do another scan to uncover more. The Intermediate scanner has a bigger range and the Advanced scanner pretty much uncovers everything in the entire system. Also server lag can affect the scans at the moment, so you may find you need to give it another pass or two before it properly fills in all the information in your nav window. Also planetary scans can often end up having to repeat once or twice too.
Doing a blanket scan with one of the Discovery Scanners will net you a basic payout for all the planets and bodies it has found. Scanning the bodies after that with only a Discovery Scanner equipped to fill in the "Unexplored" tags with data will
not net you more money. You will only get extra money for scanning the bodies if you also have a Surface Scanner equipped, which will give you additional data on the planets as well as extra income. The smaller the celestial body, the closer you have to get to it before it can be scanned. Also the closer you are the faster a scan will complete. Asteroid/Debris fields do not pay any money at all for being scanned, so only bother with them if you are a completionist. Also Ice Planets are often so numerous and pay so little that it's often better not to bother with them, unless you really must scan every last planet in a system.
Use this as a guide to give you an idea as to how much your scans will be worth.
http://lavewiki.com/_media/lw01dg5.jpg Note, undiscovered scans give a 50% bonus. You don't really need to get more than about 300-400ly out from the core before you'll start finding loads of undiscovered systems, as long as you don't fly out towards one of the main tourist destinations.