Wearing functional clothing at the gym is very important, but keeping it fashionable at the same time is not a bad idea. With a nice gym attire, you can focus on more important things - like watching your form
The best bet here is to wear simple cotton gym shorts that don't go beyond an inch or two below the knees. Wear them slightly below your waist, but not hanging off your butt.
Wear properly fitting T-shirts made out of light, breathable material, such as cotton. Wearing polyester is a blunder. Avoid T-shirts that are too large or too tight. Try keeping away from the basic white T-shirt as white does not radiate heat as well as darker colors, making it harder to stay cool and also it picks up dirt and stains easily.
As a norm stick to white, cushiony socks, or may be slight variation of white. Colored and patterned socks are not supposed to be worn in the gym
Lastly, never wear any kind of shoe except for a sneaker because you won't get the appropriate type of arch support you need during a rigorous workout.