When you buy over $100 worth using Coinbase after setting up account with this discount code:
https://www.coinbase.com/join/59341f20bb079965d35582c8
Coinbase is perhaps the biggest and the most well-known place to buy cryptocurrency easily on the web. You can pay by bank transfer of by credit card. If using credit card, you get a maximum credit card limit of $500 per week but after a while this will be increased to $2,500.
It costs more to buy using credit card than with bank transfer - but it is instantaneous if you need your coins straight away.
Don't forget that you are better off not storing a large amount of money in your coinbase wallet, I transfer mine out to my cold wallet (trezor) and keep them there, but you can also open a bitcoin wallet such as https://blockchain.info/wallet/#/ - it's super quick and easy and you can have an app on your phone too. They are the biggest and they have over 14 million wallets!
It's really as simple as that, you sign up with coinbase, buy your bitcoins, keep them in your own bitcoin wallet and then you can spend bitcoin anywhere that accepts it!
Happy Bitcoining ;-))