Hi i'm 21 i just want to know if waist training would be a bad thing to do ?

Depends on your goal: Waist training isn't necessarily a unhealthy thing to do, but it can be harmful if not done safely. If you put excessive pressure on your stomach it can be damaging. You will likely derive the most benefit from good nutrition, diet, and working out in a way that strengthens muscle areas (in this case the gluts) that you want to accentuate. Avoid fad diets and sham exercises. Have reasonable goals