How to get Vietnam visa on arrival??

There are 3 options...
1) Get a visa from an Embassy or Consulate of Vietnam in your country before you leave. This can be done in person and usually by mail as well (depends on your country).
2) Use the official Vietnam government online e-visa service. You can only get a 1 month tourist visa this way.
3) Use a visa agent to provide you with an Approval Letter. Then use that to obtain a visa at the airport.

Perhaps you can provide some more information. It will make it easier to tell you what option is best.

Seeing as you asked about VOA, just type in Visa On Arrival in Viet Nam and pick one of the various sites. Like this one, https://www.myvietnamvisa.com/vietnam-visa-cost.html

I've used http://www.vietnamvisapro.com/ twice and they are pretty good. After I filled out the form, I usually get the approval letter in PDF format in 2 days or so. They also have a service you can add that a person will be waiting for you when you land and will take care of the VOA process ahead of others that do not use the service. I'm sure they have some deal with the VOA office to expedite their customers first. There's also a taxi service but not sure if that's any better than just getting a taxi outside the airport which is pretty much what their service is.