You can buy bitcoin from individuals that own bitcoin as well as sell them to anyone interested in buying them. The most common ways to acquire and sell bitcoins are through exchanges.
We highly recommend Coinbase.com (largest) or BitStamp.net to purchase bitcoins quickly for cash. You can also 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 anytime.
The best way to acquire bitcoin is to perform a service or offer a product for sale and accept bitcoin as payment for your service and / or product.