Adder is a good starter miner. medium size (2) mining laser on top, extra room inside for refinery AND cargo. You only need 1 mining laser, and size2 generates more rocks per asteroid
Go out and do some exploring. Get away from the populated areas. those are mined out. rocks will only give like 10% on average
You want to look for metallic resources. not metal rich.. metallic. Find a ring or cluster with "pristine" resources. those give the most % rocks (40-55% average)
Have a good refinery with like 4-6 hoppers. It costs more, but will result in less wastage in dumping
Look for gold, platinum, palladium. focus on those, collect.. but jettison, the less valuable stuff. consider keeping silver and indium, if pickings are scarce
Sell this metal directly to an industrial world hungry for those metals... use the galaxy trade map overlay to see where all the gold, platinum, palladium is being sent to, then go there.