Just target the body and the scanner automatically gets to work. It'll give you a level 3 scan which will tell you what you can expect to find on the surface - just hit the system map, select the body and you'll get the breakdown in the details tab. Only if it's a landable body mind you.
This.
So it makes no difference to you once on the surface, it just helps to find a planet with the right materials. Also the DSS will get you more credits for exploration rank.
Just to add, you have to get close enough to the planet first, before the DSS kicks in automatically. Fly towards it, say with about 7 or 8 secs showing on approach speed and it will start as you get close. For larger bodies, such as gas giants and stars, you can be a lot further away when it starts.
Remember also to click on the info option on the left hand side of the System map screen, highlight the planet/moon you have scanned and scroll down as the materials available list is right at the bottom.
If you are looking for a specific material. Then not only does the DSS let you know what type of materials are available on that planet i.e. arsenic it lets you know the percentage of the material you are likely to find. So if you are specifically looking for arsenic then it would pay you to go to the planet with the highest % available.