Ah, I see. New to the Horizons?
You need an SRV. If you don't have one, they sell the hangars almost everywhere (it's a normal internal compartment of class 2, 4 and 6 for 1, 2 or 4 SRVs) Buy it, buy a buggy into it, and go land on some planet. If you check the system map, the planets you can land on have a blue halo around them. Choose a metallic or high metal content planet (not that it matters that much, here, like I said the things you're looking for are very common) and land on it. Look down on your SRV screen, and deploy. Once on the ground dismiss the ship via the same screen.
Then only drive around and watch the wave scanner above your normal radar.
Look for the signals in the lower half of the scanner band (those mean rocks - the upper half means artificial object (cargo, crashsite, etc.) ) When you see a signal, head in that general direction. at first the signal will be weak and kind of stretched over the scanner, but as you will be getting closer it gets sharper and narrower. When you are really close you will see a rock that looks a bit different than others and you can target it (it will also show on your radar as a white dot, just like things to collect in space)
Target it and shoot it to pieces. Open your cargo hatch and collect the ones you want (You need to have them targeted in order to collect them and then just drive over them with cargo hatch open)
Repeat with more rocks...