Depends on the skill level that you are starting on. If your are starting more or less new then I cannot overstate how useful a teacher can be in getting things right from the get go.

Generally speaking you want to start out with training basics like correct breathing and how to create a tone correctly. I'm the end singing is a very physical thing. You learn to use the muscles in your mouth and throat properly.

Also things like hitting the right pitch and rhythm are a thing. Depends if you have a musical background otherwise.

And very important: If something hurts die out after singing, you're not using the right technique. It will occur that your do things wrong while practising but listen to your body and don't overdo it.

I can warmly recommend Catherine Sadolin's Complete Vocal Technique. It is a fantastic book especially if you don't want to / can't get a private teacher.

I hope this was somewhat helpful to you, cheers!