A motorcycle gas tank is basically what it seems like, a big empty compartment for storing your gas. However there are a couple of things to remember when restoring one for the first time.
First, the outside. If there are any dents take care of them before painting! I have a bunch of videos on dent removal and (amateur) paintjobs. Here they are:
So, that is how to make the tank all nice and pretty on the outside. However, far more important to the running condition of the bike is how to make it clean on the inside. First, dump out all the old gas if it is older then 6 weeks. See what comes out. Bits of junk are fairly common, a lot may require a cleaning.
Assess the cap as well. If it is an ill-fitting cap, or the rubber gasket is cracked, you may easily get water infiltration and that needs repair. Also, there should be one or more breather holes around the gas cap, to let air in as gas flows out. If these are clogged you may potentially get fuel starvation problems.
As for tank lining, the only product I ever use is POR-15, and even then I only use it when a tank has pinhole leaks. Kreem gets nasty quick and is almost impossible to clean out. I maintain that the best way to keep a tank rust-free is to USE IT. That means new gas in, new gas out as often as possible. Ride your bike! Keep the tank filled with gas when it will be sitting for over a month. That's basically all there is to it.
Here are some videos on rust removal from your tank: