How do I train my singing voice?

My advice will seem contradictory to your question but the best thing to do is work with a singing teacher! Training your voice independently, although it can be done, is very challenging without the correct knowledge and understanding of vocal technique and anatomy. Working with a teacher will ensure you are not causing any damage to your voice by using improper technique or even singing dangerously (it sounds dramatic but the vocal chords are easily damaged, it is possible to cause permanent damage to your speaking voice). A good teacher can spot things you’d never notice yourself, correct bad habits early, and guide you step by step. If you’re practising alone, it’s easy to build tension or reinforce mistakes without realising. I would say you don’t need to know if you are already a ‘good’ singer before you start lessons - that’s what the lessons are for! All you need to know is if you enjoy it and if you want to sing!