How could I make my t-shirts better?

It looks nice but I think you should work on layout/ balance... the top of the design doesn't look balanced with the bottom... plus make sure the colors work well with each other. Keep it up!

Improve the font, it looks kinda pixelated, looks like an xbox 360-ps2 tokyo game, if it is the vibe you wanna give, then it's GREAT, but if it isn't look forward for a font with more definition, also the smoke looks kinda "meh" because of the black background, maybe a fade would improve it a lot.

I would lighten the amount of smoke to the design while centering the top text more then tweaking the side text down a size. The effect on the chrome looks a bit choppy on the edges and the texture look the Japanese writing needs some work. I guess it really depends the look your going for as printing would be pricey if it isn't DTG but a more simple color scheme would look good on this design (ex: single colored main text, Japanese text a different color, while having the dragon in similar color scheme)