If the tank is just dirty, or maybe even some light rust, drain it and take it off the bike. Then put a quart or so of kerosene in it with a hand full of nuts, bolts, etc., put the cap on, and literally shake the crap out of it. Then drain it out, rinse it several times till everything comes out clean and put it back on the bike. Don't forget to clean out the petcock and change the fuel filter. You may want to change the fuel filter every few tankfulls for a while. If it's really rusty inside, you probably need to buy a kit and cream it. That coats the inside of the tank with epoxy like stuff to seal in all the rust.
POR-15 is one of the most highly regarded kits for derusting a tank. If you get it cleaned at a radiator shop they may put a liner in it or just use POR-15 US Standard Tank Sealer. You can order their products right off the website.