A lot depends on what type of singing you wish to do. Some people do naturally have a "loud" voice" but there are a number of different techniques which can help to amplify your natural volume level.
The secret to most techniques starts with correct breathing. If your lungs are being squashed due to your body position - slouching forward, hunched etc. you will not have enough air in them to allow you to project, or to hold a note. If you are bending your head down to look at your chest (singing into your boots), you will be constricting your windpipe, and you will also not be able to sing as loud. Both of these can often be combined with lack of confidence.
Sit or stand up straight, with shoulders back and note the difference in the shape and position of your rib cage and chest.
You can pull air into your lungs by expanding your rib cage, but this doesn't produce as much control as using your diaphragm (the layer of muscle which separates your chest cavity from where your intestines live. Breathing from "the stomach" is what enables singers, brass players etc. to hold and control a long note. It also allows you to sing/play a longer number of words/notes before you need to take another breath.

Once you are breathing properly, what you do with a note is changed by different factors. We all have a personal range of notes we can use for singing. Some people have deeper or higher voices. Others have a wide or limited range. I am sure that the "trained" singers will tell us that if taught correct techniques, it is possible to extend your own natural range, but that is beyond my personal experience.
I have discovered that certain parts within the range of notes I can sing are "stronger" than others. Also that a song which sounds fine sung realtively quietly at home will disappear into the background noise when sung out. Often this can be overcome by starting a couple of notes higher in the scale, where the natural resonance is slightly different, but holds up better in a larger air space.
(must dash - might give more thoughts later)
Quack!
GtD.