Good haircut/hairstyle (as suggested by NewBeginnings, above). Why dye if you don't have to? Natural can sometimes look much nicer.
Properly fitting bra. Stand up tall. Breathe well. Gentle exercise can boost confidence: make you feel 'good in your skin' as the French say.
Good grooming: nails, skincare, shoes cleaned, clothes mended/aired/ironed if needed - there were several strands about this in Style and Beauty. Don't know where they've gone?
Elegant makeup - as NewBeginnings also said, the consultants at department store make-up counters can be very helpful. Or you could ask at local Further Education college about cheap practice sessions for facials/makeup by students.
Subtle scent - many strands here on Stlye and Beauty; many with expert information.
If you can afford it, think about going for a consultation with House of Colour or Colour Me Beautiful. They will help to choose colours that suit you, as you naturally are. Plenty of strands about this in Style and Beauty. Both organisations have websites; they might give you some pointers.
Look at Pinterest for ideas about clothes. Then buy carefully. Better to have just one or two outfits that really flatter you than lots of cheap garments that don't.
If you have the time and patience, then visit charity shops, jumble sales, ebay etc for secondhand/vintage items.
Be VERY careful about cheap jewellery - unless it's made from 'naturally' cheap materials, eg a necklace of glossy wooden beads, or a bangle of clever 'plastic amber' . There are some good cheap things about, but IMHO, not that many ... With jewellery, often 'less is more'.