You can buy it from an exchange or a person or a Bitcoin ATM...
To buy from an exchange, you would need to create an account on an exchange (Go and search Google for Poloniex, Bittrex, Kraken, "bitcoin exchange" etc). They'll probably want you to fill in a load of personal information due to KYC/AML regulations. If they support it, you can then wire them money which they will deposit into your exchange account and you can then buy some bitcoin. You then withdraw that bitcoin to an address in your Bitcoin Core wallet
To buy from a person, you can use a service like localbitcoins, or goto a local bitcoin users meeting, or just ask random people on the street if they have bitcoins!
You would then meet up, you give them your Bitcoin Core wallet address, they send bitcoins to your address, you give them cash.
You can also trade with people online using services like Paypal or bank transfers etc, but that can be quite risky and there are a LOT of scammers and thieves... using a trusted escrow service is HIGHLY recommended.
Bitcoin ATMs are pretty nifty. You basically insert cash and it will print out a paper wallet with the necessary information (private key) to transfer the coins to your Bitcoin Core wallet. You then import that private key or sweep it into your wallet.