Keep in mind that the Bitcoins are always on the blockchain, not in the wallet. The wallet is only the holder for the keys. And the keys are the (only) things that allow you to spend (they tell that the coins are yours).
This means that if your computer is compromised and the wallet file is copied by somebody else, or somebody can record your cations on the computer, or, or, ... there are plenty of ways your coins can be stolen if you are not careful enough and don't know what you are doing.
(This also means you should not store your wallet, password, seed on cloud or e-mail, for example).


The only way to mine Bitcoin is with special machines called Bitcoin (or SHA) ASICs. They are not cheap and they consume electricity, meaning that only if you also have dirt cheap electricity (if your parents pay for it that doesn't make it cheap or free) you can mine at a profit, else it's cheaper to buy Bitcoin.