To start off mining, you have to make a pickaxe. If you do not yet have pickaxe and do not know how to make one, the crafting guide can be accessed here. Once you have your pickaxe, you can now start mining. Now before you start mining there are some things to remember.
Also, with mining you may come across randomly generated places. These can include strongholds, mineshafts, ravines, and dungeons. These places, have absolutely nothing in them. They are just filled with booby traps, so if you ever come across these, just build back up to surface, since these are a waste of time.
First off, you get your fists out and mine directly under you until you reach stone, once you reach stone just whip out the secret weapon. The wooden pickaxe. Keep on mining as far down as you can until your wooden pickaxe breaks or you see any type of ore like iron, diamond, emerald, redstone, lapis, or even gold. If your pickaxe breaks, just go back up and make more wooden pickaxes. Once you get down to see these ores, is when the mining trip finally gets worth it, you mine any ore that is yellow so you know its gold. You might not see an item drop from it, but it just goes straight to your bank account. All you have to do to access your bank account is beat the Ender Dragon 3 times. It is pretty simple. But back to mining, you just keep mining straight down and get every gold block and avoid every other block that isn't stone or gold. When you get to bedrock, you finally made it. If you mine a bedrock, you get all the riches in the Minecraft world. The thing about bedrock though, is that bedrock takes a very long time to break. To break bedrock, you need to mine it with a wooden pickaxe for about 30 minutes to a couple hours. Bedrock has a 1/100 chance to break though, so you need to block at least 100 blocks for at least 2 hours for to get the prize. You never know which one it is as it is completely random. So with that said, good luck on your mining journey!!