Someone posted a guide for this on here a few weeks back, I copied it. It was genius..
Firstly, pick your favourite neutral and stick to it - so ideally navy or black, or it could be brown or grey. EVERYTHING you buy has to go with this. You can branch out into additional neutrals later.
Then buy yourself:
2 or 3 pairs of jeans
A pair of smart trousers - warmish
2 casual jumpers / sweaters
A denim skirt or pinafore (or both)
4 well-fitting short-sleeve tshirts
A light denim or other light shirt
3 or 4 long sleeve tshirts / polo necks
A belt
A cardigan of some sort, short enough to go under the coats
A weatherproof coat
Some new socks
Some casual trainers / pumps that go with everything
A couple of pairs of nice thick tights, try knitted, something with a bit of colour
A pair of flattish ankle boots that go with everything
A smarter, longer coat
A large thin scarf
A chunky winter scarf, hat and gloves
A casual handbag
A smarter handbag
A couple of pieces of 'funky' costume jewellery
Everything MUST fit and match. Don't worry too much about the quality for now. Aim to get all of the above over 2-3 shopping trips (NOT online!) - try everything on and only buy things you feel comfortable in straight away.
Wear all of this over the autumn / winter and then gradually replace with things you like better / seasonal pieces / better quality items or as things wear out.
Like a PP has said, get a good hair consultation.
Get newer up-to-date glasses if you wear them.
Take EVERYTHING out of our wardrobe and ONLY put back things that you feel good in, will definitely wear, and are in good condition - get rid of everything else. Put out-of-season stuff away till next year.
Just do a small amount of upping your game on skin and hair - e.g. some sort of style everyday (mum bun for cleaning / gym only), moisturise, minimal makeup / lip balm, look after your nails etc.