Select from any one of the Exchanges above and sign up for an account. It is free. I recommend Coinbase.
Note: When you purchase Bitcoin from any one of the Exchanges listed above, you will be provided with a Bitcoin wallet. Your Bitcoin wallet stores your private key, a secret number—a 256-bit string—that gives you access to your Bitcoins. Your private key also allows you the freedom to move across the crypto-market place and enter into the crypto-Exchanges such as Bittrex, Binance, Kraken, Poloniex, Cryptopia, Coinexchange, and many others to purchase Altcoins with your Bitcoin.
How To Buy Bitcoin (Coinbase)?
The video below shows you how to purchase Bitcoin via Coinbase.