You can buy bitcoin, ethereum, and litecoin from individuals that own bitcoin as well as sell them to anyone interested in buying them at www.LocalBitcoins.com. The most common ways to acquire and sell bitcoins are through exchanges.
We highly recommend Bitstamp.net and Coinbase.com to purchase bitcoin, ethereum, and litecoin using your bank account or credit card. You can use Local Bitcoins to meet with buyers and sellers in your area and purchase bitcoin directly. You can purchase coins from these companies and also sell coins for cash to convert your coins back into your local currency at any time.
The best way to acquire bitcoin is to perform a service or offer a product for sale and accept bitcoin, ethereum, and litecoin as payment for your service and/or product.