The most important aspect is your body. Someone in good shape can make clothing of the worst quality look expensive.
Then comes fit. Different for everybody, just as important as fitness. Well-fit clothing on someone out of shape looks better than poorly-fit clothing on someone in shape.
Not every style is meant for you. Imagine James Bond in baggy jeans. Figure out what works for you; good fashion is ultimately a test of your ability to ignore trends and dress according to who you are.
The other details–color, material, quality–are an afterthought if you don’t consider the above.
Dress well by staying in shape, finding clothing that fit you well, and determining your style.