Of course anyone can learn to sing, even the "monotones' mentioned above. The question really is: Can anyone become the kind of singer you would pay money to hear? The answer to that, unfortunately, is no. The solution is: sing anyway, even you monotones. The first reason to sing is to please yourself; if you can please others too, that's nice but unnecessary. After all the great advice given above, the only thing I can add is to cite Stravinsky: "It's not enough to hear. A duck can hear." Listen! Listen to yourself the same way you listen to others. You may not be able to equal them but you can certainly improve.