GTA:Mining
Getting Started
To start a mining run, you need to prepare prior to going. You'll need to secure a pickaxe, which you can purchase from a General Store. The mineshaft is located in the middle of the map, and south of Fort Zancudo. Upon entering the mineshaft, you will be able to find multiple places to mine materials, the deeper you go the more chance you have for valuable stuff. Go to an ore and stand on it, you will get a notification that you will be able to mine. Then press Left Alt in order to start mining. Once your inventory is full, head to your car and press Y to place the goods inside of your vehicle. Once the car is filled, you're good to go to the processor. There is only one mineral that does not need to be processed.
Processing Minerals
Once you're finished with filling your pockets with ores, you have to process what you collected, this location is marked on the map, in the southeast of the city. Head inside of the factory and to the large cauldron. Here you will find an area with floating text prompting you to press Left Alt to start smelting, this will open an inventory menu, at which point you can drag the mineral and begin processing.
Selling Minerals
Selling minerals is done entirely through other players. There are entire groups dedicated to the buying and selling of materials in addition to independent players buying them as well. You will need to put in the leg work through Tweedle and connecting yourself with other people in the mining community, with great effort will come great reward.
Experience
Different minerals and materials take experience to work with safely and successfully. You won't be able to mine the rarest minerals on the island immediately and to do so will require time and effort on your part. You also can't be an expert in everything! Mining is a full-time job and so is knowing how to operate a foundry, this is where it helps to have friends or business partners.