You have to control all the tiny muscles in your throat, face, jaw, and mouth. Learning these concepts and training yourself with practice will not only help you sing better, but it will also improve confidence in other areas of life where you feel bad. Starting to Sing is an audio-enhanced e-book that is different from all the “singing for beginners” material out there. It doesn't mean you can already sing in tune or have control over your voice.
Instead, it starts from the basics and takes you step by step to learn to control your voice and sing with precision and reliability. In more than 100 pages and with dozens of audio examples, Starting to Sing shows you how to break each of the 5 barriers and quickly become a capable and confident singer. Getting started singing is for beginning singers who are looking for a fairly comprehensive overview of all aspects of developing a pleasant singing voice. Honestly, for bad singers I recommend personal voice lessons.
You really need someone who can focus on your specific problem areas and work with you from there. Anyone can give generic singing advice for bad singers, but a personal vocal trainer can give you detailed tasks to improve your problems and can give you instant feedback on whether it's working or not.