This is a slightly advanced topic but we will keep it as simple as possible.
For USA / United Kingdom / EU countries:
The easiest way is to use an online "wallet" or account manager like https://www.coinbase.com . This is actually who we use to receive payments as well so the process is much quicker than other ways. To start you will need to register on Coinbase. Registration is simple and quick. Once registered the steps vary based on your location.
For U.S. citizens you can find out what to do here: https://support.coinbase.com/customer/en/portal/articles/1148716-payment-methods-for-us-customers
For U.K / EU citizens you can find out here: https://support.coinbase.com/customer/en/portal/articles/1767231-payment-methods-for-european-customers
One option we have found is Zebpay: https://www.zebpay.com/
Some others: https://www.unocoin.com/
We however do not recommend or endorse zebpay or unucoin. We have never used either of them. If you have, please let us know how your experience has been.
We have found two options: https://vbtc.vn/ and https://www.bitcoinvietnam.com.vn/We however do not recommend or endorse vbtc or bitcoinvietnam. We have never used either of them. If you have, please let us know how your experience has been.
Worldwide (including previously mentioned countires):
If you cannot find an online site to buy bitcoins coins you can always try offline. You can find local sellers of bitcoin on https://localbitcoins.com/ . You will usually meet in person and exchange there. Please be careful as you will be dealing with individuals and not companies! You will need a wallet before purchasing. To our knowledge you can use Coinbase for that regardless of country you live in. The only restriction is buying and selling through Coinbase. Sending should not cause any issues. If you would like to find out how to make a wallet, see here: https://support.qqtube.com/support/solutions/articles/4000096115-what-is-a-bitcoin-wallet-and-how-do-i-make-one-