Just like your everyday clothing, your success in being stylish at the gym lies in the small details of your outfits.
1- Add some color; a good option is to get colorful sneakers (pict.1), it definitely will give a special touch without been too obvious.
2- Get a cool backpack! (pict.1,2,3)
3- Use layers (pict 2), show off your top. Try cutout and cropped t-shirts.
4- Go shine! Don't be afraid... it's super in right now. Leggings, shorts, tops, t-shirts... just remember to wear just one (or small pieces) at time!
5- Leg warmers (pict 3), a sexy 80's touch for your look.
And last but not lest, be ALWAYS comfortable and be YOU.