Decide on a base neutral colour that suits you. It might be black/grey/navy or warmer shades of brown/taupe/oatmeal depending on your colouring.
Think about what's happening in your life for the next three months: any big events you have to dress up for? If not, concentrate on everyday and ordinary social stuff, to cover work and play. Ignore holidays for now.
Buy a couple of fashion magazines and look through them, several times: Elle and Vogue perhaps (but not celebrity gossip magazines); which images take your fancy?
Then consider what reasonable towns with a decent range of shops are nearby? If you don't go there regularly, plan a day out (best not at the weekend unless you've no alternative). Wearing jeans and a plain T shirt, start by walking around and looking in the windows. You will like some more than others: start there. Just go in and pull a few jackets off their hangars and try them on. This doesn't even involve changing rooms. Do you like what looks back at you? Does a particular colour make you look well, and your eyes sparkle? Does one shape work better than another, eg single or double-breasted? Put it back, and walk along a bit more, then repeat the exercise in another shop with different colours and shapes.
You don't mention your underwear, but at this point it's a good idea to try on bras (you need two or three in nude and/or black). Don't buy any before you've pulled your T-shirt back on to see how you look dressed wearing them (this is important, as it needs to vanish under clothes, making the shirt look better: it's my reason for suggesting you go out in jeans and T-shirt, because that's your comfort zone).
Try on more jackets, and if you've found one that fits on the shoulders, fastens comfortably and flatteringly without pulling across your bust, look for a skirt and trousers in the same colour, or a colour that tones, and a blouse or shirt in a pattern that includes the main colour and some lighter/brighter/other colours.
If you are still reading, and have ticked all these off, you might like to buy a few new T-shirts/vests in the colours you've selected so far. If your base neutral is grey or navy, go for those, plus white and something else.
Next, I would look for a dress, possibly cotton poplin (summer is coming), maybe a midi-shirtdress that buttons from collar to hem, in another pattern based on the colours you're liking on yourself. You should be able to layer the dress over all the plain T shirts, and put jeans/leggings underneath like a thin coat, or wear it alone, buttoned or unbuttoned as much or little as you like. If it has long sleeves, you can roll them up or down, and you should find a coordinating belt to define your waist. A simple pair of pale sneakers, and some jazzy earrings. If you manage all that, you're well on your way to your new capsule wardrobe! You would be lucky to get it all in one shopping trip unless you stumble across an independent shop and a salesperson (often the owner) who has a very good eye and time to help you. A personal shopper somewhere like JL or Debenhams might also take some of the pain out of it.
There is a link to a useful illustration of a capsule wardrobe in the how to look polished thread, about 100-150 posts back IIRC, with lots of pictures. It's mainly US stuff, but there's lots of sound advice.
I've spent ages thinking and writing this, and someone else is bound to have said similar. Best of luck OP, and I wish you success.