"In minable zones, shoot the small floating asteroids to find valuable commodities" is how the game phrases it. But this can be easily missed. First, you must be in a minable zone. On your NAV map (F6), click on Minable Zones Map to see where you can mine. Proceed to one of those zones and you'll find a bunch of asteroids in which you can destroy to reveal a commodity to tractor in. You'll know what asteroids can be shot down because when you hover your mouse over them it will turn into a revolving white circle.
Note that this is somewhat tedious as a lot of mine fields have "empty" asteroids. Others are very rich with elements and commodities. Check the money tips section and you'll find some tips on specific places to go for good mining.