For something on the off-duty side, wear this combo of a white and black crew-neck t-shirt and shorts. And if you wish to easily spruce up your getup with a pair of shoes, introduce a pair of tan suede loafers to this look. You can take inspiration from ensembles like this if you're a teenage guy and you want to take a more serious approach to your personal style.
White and Black Crew-neck T-shirt with Shorts Smart Casual Hot Weather Outfits For Men In Their Teens
For a look that provides function and style, consider wearing a white and black crew-neck t-shirt and shorts. You know how to dress it up: black leather derby shoes. This ensemble is most probably exactly what you've been looking for as a teenager.