Alright, so you just graduated from college. You are ready for the world to take you seriously. Yet, your visual perception (your clothing) says you are not ready for seriousness. You do not want to loose your casual style, so you are reluctant to make the wardrobe change.
You can still keep the polo or button down with jeans or khakis. You just need to step into the mature version of the casual style.
Instead of wearing baggy clothing that exaggerates you did not lose the freshman fifteen, choose clothing that actually fits your body.
Make sure the line where the sleeves and shirt meet hits where the shoulder and arm meet. Allow the shirt to touch your torso instead of being too wide that you look like you wore a tent.
Choose a straight leg or skinny version of jeans and chinos. Refrain from buying jeans with holes or washed with some weird pattern. I think dark jeans make you look more put together. Stray away from light washes.
You can still wear sneakers, just scrap the high tops. Choose a basic, classic sneaker. You can even ditch the socks too. If you want to step up your game, wear boat shoes.