You can start mining at any level and as soon as you create your account. You can mine the 'community rock' located in the cave. Alternatively, you can find smaller rocks which spawn every 2 minutes at the Dragon's mine and Fang's Mine areas. You do not need to purchase a pickaxe to begin mining, you already have one. If you want a better pickaxe you must upgrade it by visiting the trader who is located inside the Crystal Cave.