Record your voice singing Don't Cry For Me Argentina (if you're a girl), Daddy! Daddy! Do me! (if you're japanese or a weeb) or any Ar Tonelico 1 or 2 songs if you just want to practice vocals. Don't bother with Art Tonelico 3, the vocals and songs were shit, only 2 were what Ai-chan consider any good. Then listen to it in agony.
After you've finished feeling shocked from the cringe of your own voice, sing and record again. Then repeat the process. Until you are no longer cringing from your own voice. At that point, you have won. You may not have the best vocals, but you will at least either start being super confident or super depressed.