It largely depends on where you are based. If you're in the US, for example, the steps described by ETFbitcoin could be translated into:
1. Buy Bitcoin (BTC) from Coinbase.com using your credit card.
2. Download Electrum from Electrum.org and set up a wallet according to Electrum's instructions -- withdraw the coins from Coinbase to your Electrum wallet.
3. Send the required BTC amount to the sellers Bitcoin address.
That's a very simplified verion of what you
could do.
In reality you have a large selection of exchanges with varying trustworthiness, availability and ease of use:
https://bitcoin.org/en/exchangesDepending on your location you may even have a Bitcoin ATM in your vincinity (although the rates are usually worse than what you get from an online exchange):
https://coinatmradar.com/Be aware that when registering a new account with an exchange you'll likely have to get verified first (ie. provide them with personal data, verify your bank account, etc) which may take a few days.
You also have a vast selection of wallets for all sorts of devices:
https://bitcoin.org/en/choose-your-walletBe aware that you should backup your wallet, do not share your wallet file or private keys with others (essentially anything that isn't a Bitcoin address) and note that if your device is hacked, money might be at risk (ie. if that's more than just a one-off transaction, consider looking into cold storage and / or hardware wallets).
And especially,
heed LoyceV's warning -- there's a lot of scammers out there, so buyer beware! Bitcoin transactions are final and if you don't know who you're dealing with they might just make a run with your money.