Finding things planetside is a bit of a two-pronged assault, especially if you're looking for any salvage commodities:
Start out by descending to the planet's normal space out of Orbital Cruise. You'll want to keep your altitude above 2 km, otherwise your scanner will not pick up any points of interest. From here, you'll basically want to fly in a straight line as you keep an eye on your scanner. Purple circles indicate ground-based points of interest; use your keys bound to Increase/Decrease Sensor Scale to help track them down. Once you find a circle, go ahead and touch down on the planet.
Now that you've landed, you'll have to use the Wave Scanner to pinpoint where your future goodies hide. The wave scanner works like radar; it'll sweep across the scanner display and anything of value will appear as a different set of lines and emit a distinct tone. The farther you are from an object, the larger the line(s) will be. Lines will shrink as you get closer to them (I've also noticed that if you drive fast at an object, it will appear shorter than it actually does, like a doppler effect). When you get close enough, the item(s) in question will appear on your regular scanner. If its a rock with materials in it (Mesoderite, Bronzite Chrondite, Outcrop etc.), shoot it with your turret to extract the minerals. Then, deploy your cargo scoop, lock onto your material/cargo, then drive over it to collect it.
Once you've scooped some cargo, you'll have to transfer it back to your ship. Start by opening the bottom panel (its between the pilot's legs in VR/free look) and navigating to the SRV tab. Recall your ship (if you dismissed it or drove 2 km away from it) and head on over to where its landed. Drive under the bottom of the ship into the orange docking light. Once the "Board Ship" Status indicator lights over on the right side of the HUD, you can transfer your cargo or board your ship via the bottom panel. Repeat until you have all of your cargo or you'd like to take off in your ship.
Bit of advice for planet side landings: pay attention to your gravity meter! You'll find it under some degree readouts under the altitude bar. It'll look like a number with a G at the end of it (example: 1.23G). Anything under 0.80G is low gravity and relatively easy to land on. 1.00G is the same as Earth's gravity, its ranks 'moderate' on the difficulty scale. Anything above 1.00G is high gravity, and is difficult to land on. Keep your ship's weight and thruster rating/class in mind; gravity is an unforgiving mistress and many a pilot have faced the rebuy screen on some higher G worlds.
If you're interested in what each tone/line means on the wave scanner, pop over to
http://wavescanner.net/Good luck CMDR!