Mining

Head to The Mines on the map and speak to the ped.

They will allow you to buy pickaxes and will also buy your mined ores from you.

Once you have a pickaxe go over to some rocks within the quarry and mine them. Note that there's different XP levels so you won't be able to mine everything right away.

Once you've got some ore collected, you can either sell this to the ped or if you want to make some real money then go to the Smelting location on the map and smelt the ore into ingots. Again you can sell these to the ped at The Mines.