You will need a valid passport (at least six months before expiry) and a visa before you enter Vietnam. A 'visa on arrival' is not impossible but it does need to be arranged before, please see below. To obtain a visa you need to visit a Vietnam embassy or consulate, but it doesnt have to be in your home country. For example, if you arrange a visa in Thailand its around $50, but in the States maybe $85.
Tourist visas are good for one month and can be extended once in Vietnam, for a fee of $25. It is possible to get 3/6 month tourist visa but you should speak to the embassy directly. You should ask for a multiple entry visa if you are planning on taking a trip to Thailand, Laos or Cambodia.
Please click here to see a list of Vietnam embassies worldwide.
You may find that the embassy in your country is very far from you, but it should be possible to apply by post. If you wish you can use a Vietnamese travel service on the internet, who will arrange an electronic 'visa on arrival' for you. You will have to pay them, and then a fee at the airport (from $25). Take a look at these companies:
Citizens of Scandanavia, ASEAN countries, S. Korea and Japan do not require a visa but may only be valid for 15 days, so you will need to check.