Fashion is a form of anything, it can be a style, a shape, an appearance, or mode of structure. It is also a pattern, a model, a workmanship etc... But don't you think that to be in fashion is just to be yourself. It is not only wearing designer clothes or collecting fashion accessories from top fashion designers.
The real meaning of fashion is wearing something that can make you happy, that can make you comfortable. Fashion is temporal; style is always "in". You can buy the former, but you can't buy "style" - either you have it, or you don't. Some may even learn to cultivate it.
Remember that the secret to look good is to feel good about what you do and what you wear. So even if you just wear polo, t-shirts, pants or shorts. Just choose a simple yet elegant good looking clothes.