Can you print a certificate of a bitcoin you own?
No, but you can print the cryptographic keys associated with the address where that BTC is held and it could be used to transact.
There are no "certificates" for BTC nor is there any data resident on the personal computers of the owners that represent the BTC themselves. All there is is a blockchain copies of which exist on thousands of computers. That blockchain is the ledger for all BTC transactions that have ever happened and the only place they exist.
How are bitcoins mined as opposed to buying one?
Mining a BTC is a prerequisite for transacting with it. All BTC that exist were mined. A portion of those have subsequently been sold, then the buyer sold them again, etc etc. You can buy any amount of those BTC you want via an exchange, or by contacting its owner and negotiating a price. Show him the above print of your public key and he can send you some, which the blockchain will prove out and make record of.
