Why do you want to sing louder? Learn how to use a mic, and you won't need to sing loudly. And singing quietly is a lot more expressive in most songs. If you really want to sing loudly, you need to be able to control your vocal apparatus, as has been said. But this is also important for singing quietly, and holding notes a long time. There really is no substitute for a good teacher, and the best teacher for the style of singing you want to produce would be someone who sings in that style. I am very fortunate in having had some lessons from Maddy Prior, who is more than capable of singing quietly or loudly and at all times with great purity. What recommended her to me was that I loved her singing and loved the songs she sings. Find a teacher who sings the kind of stuff you like to sing is the point I am trying to make.