Research: Look online at people/celebs you think are "stylish" and see what they do well. Also look at your body shape and try to wear things which flatter that. There's a guide here but you can find others online as well.
Choose a colour scheme so everything goes together. Nearly all of my clothes are black, white, grey, or dark blue which saves a lot of time in the mornings.
Choose blazers, smart jackets and leather/leather-look jackets rather than fleeces.
Choose t-shirts/blouses which fit well.
Wear jeans or trousers rather than leggings (okay, some people look great in leggings. I am not one of them)
Make sure your hair is always clean, dry, and nicely brushed (straight parting, no flyaway frizz etc). Get it cut before you get split ends.
Follow a skincare routine, including exfoliating especially if you have greasy/acne skin (like me
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I just wear lip gloss & mascara normally, sometimes nothing. Other people will wear more. I think makeup is something which you need to try out yourself and see what suits you, or maybe book a tutorial with a consultant e.g. at MAC - I've no idea how much that costs though.