it's called gas, and they have these things called "pumps" that you can drive right up and get some...
a tank won't rust if you keep gas in it, unless you let it sit for months and months. just make sure it's clean before you fill it up the first time. a piece of bike chain, about 6-10 links, will knock out any chunks of rust. just drop it into the empty tank, and then shake it up all paint-mixer style, then dump out the chain.
rinse it out with some denatured alcohol and it's good to go.
Kreem tank liner (and other brands) can be used if you want. it's great for custom tanks or older ones that may develop pinholes. it's also good if your bike's gonna sit around and not get used for a long time.
but like i said, the gas, especially a two-stroke mix, will keep the tank from rusting.