I've recently relined a tank with the help of Ian Potter from Tank Care Products near Norwich. The ethanol in petrol these days causes water to accumulate, especially in tanks on bikes that are not used much.
My tank had previously been coated with Petseal which had been degraded by the ethanol and needed removing with a solvent. Ian sells all the gear you need plus is a mine of information (loads of info on his website).
I would suggest that if you simply want to clean your tank, much of what has been said already makes sense. FYI, I wrapped my tank in many layers of cardboard and bubble wrap, put a couple of handfuls of sharp sand and grit in it, wedged it in a cement mixer drum and tumbled it for a few hours.
But I do suggest lining it. It's not hugely expensive to do, it's fairly easy if you take your time and concentrate although you can send your tank to Ian and he will do it for you. Incidentally, he is also one of the best in the business if you want your tank painted, folks send him tanks from all over the world.
One last thing, the only petrol on sale in the UK with no ethanol in it is Esso Synergy +99, except in certain parts of the country because it already has ethanol added at the relevant refinery.
Incidentally it's very easy to remove ethanol from petrol: mix it with water, let it settle and drain the water off which takes the ethanol with it.
Ethanol in fuel is another one of those things with great intentions but not so great in practice........