Look for patches of earth that are lighter brown than the regular texture and loks like it has stones mixed in . Strike it until you get resources from it. If you get more than just clay dirt, grats you found the gravel that signals or veins . Also, pay attention to what resurces you got from that spot, as it will signal what you can find below.
Often times these are best fund between grouping of small boulders ( the ones that give you raw iron) .
Dont pay much attention to people teling you exactly how many blocks down to dig, because it varies on elevation ( and i have no idea how to see at what degree i am above or below sea level) . Just dig down until you have about 3 or 4 blocks of stone overhead, then dig shafts in the four cardinal directions . If you dont find any , either move on or go deeper .