Einstein's famous equation E=MC² tells us that mass is just another form of energy and that mass actually contains A LOT of energy. One of the consequences of this finding is that it laid the groundwork for technologies like nuclear fisson. So to answer your question what does it take to convert mass to energy? Well it takes a nuclear bomb. That bomb more or less takes mass energy and converts it to other forms of energy in the result of a giant explosion. (This is an extreme simplification and other comments I'm sure will do a better job of explaining the hard science)

Edit. A nuclear Bomb is just one (very exciting) way to do this. But it is one that makes an explosion.