Modern Vespa

When one purchases a brand new tank that has oil all over it, what do you do to get it ready for installing it and running it with gas?

Do you just wash it out?
Need some kind of chemicals?
Throw it in the river?
Light it on fire?

I would blow out inside with compressed air to ensure the factory didn't leave any surprises. Then rinse with running alcohol or gas until I gain confidence there was no foreign substances/dirt.

If it came with any accessories (petcock, etc) I would tear down/inspect if possible. I have had mixed luck with "new" parts being 100% ready for use.

Wash it out with gasoline and inspect should be more than adequate, it's basically loose dirt and gasoline solubles that you should worry about. And as Captcha said, inspect everything if something comes assembled, tighten everything down just to make sure etc. Also, check that everything is leak free before you put the tank back in, at least on my PX any gas that leaks around the fuel tap will eventually drip down on the exhaust.