So dressing fashionable comes down to three things, Fit, colour and style....
Fit: google up on your size and find what looks best with your body shape... the same clothes wont work for a 5 foot 4 80kg person and a 6 foot 65 kg person....
Colour: what goes best with you're eye/skin colour... also learn the colour rules with clothes... for example never where grey pants/jeans with a grey hoodie/ top because you will look like you just got out of prison..
Style: look in magazines, what are celebrities wearing and what is advertised in clothes store windows and go with those styles....
DONTS: trackpants (always go jeans), Sneakers (leave them for running), Denim jackets (leave it to the cowboys)
DOS: Jeans and tee (best casual way of dressing works for anything from a casual date to watching tv at home), Over dress for good occasions (I always wear a colared shirt tucked into black pants with a nice tie for family events, dinners, weddings, 21sts etc etc), Plain shorts (easier to match with a tee shirt than board shorts in summer)
FINAL AND MOST IMPORTANT TIP: CONFIDENCE! whatever you wear, wear it with confidence.... sometimes you could be the only one at an event dressed in a shirt and tie but if you pull it off with confidence you will be turning heads... as my mumma always said stand up straight, dont slouch your shoulders and look people in the eye... GOOD LUCK!