I would start with eyebrows. They can have a HUGE, HUGE effect on the overall look of your face. (Don’t believe me? Check out my face in 2006 vs. my face today.) Go to a professional and have them shaped. This usually isn’t too expensive – I paid $5 for my initial shaping, and it was at a really fancy salon. Then get a pair of tweezers and just maintain the shape that the pro gave you.
Next: makeup. Some things I consider basic and essential are eyeliner, mascara, foundation, concealer, and at least a couple of different lipsticks or tinted lip balms. Keep in mind that you definitely don’t need to wear all of these things every day – I certainly don’t – but they’re good to have around. I’d also pick up an eyelash curler to use with the mascara, and a few brushes for makeup application.
Hair – see a stylist you trust with your (hair’s) life, bring pictures of styles you like, do some research on what styles and colors would be flattering to your face shape and skin tone, and dive in. (FYI I used to be a blonde and my look gained soooo much impact when I went brunette. I’ve been going progressively darker since that initial dye job and I love it.)
Clothes – make a style statement, or a vision board, or any kind of fashion-y compilation that helps you picture where you want your style to go. (For example, I have one on Pinterest and I also have a physical notebook where I glue in inspirational magazine clippings.) From there, make a shopping list of items you want to add to your wardrobe. (e.g. orange cardigan, blue dress, green tights, black boots…) Then go shopping. If I were in your situation, I’d probably save up money for a while so I could go on a proper shopping spree, and/or just check out thrift stores.
Then I’d start taking outfit photos on a daily basis to monitor how different types of clothes looked on my body. This is extremely useful information.
Some people’s first inclination in your situation would be to go out and buy a stack of fashion and beauty magazines to find tips. I think the information on the internet is often better and more specialized. You can type anything into Google and find it within seconds. “How to do pinup makeup?” “How to dress like Cyndi Lauper?” “Where to find blue leather heels?” It’s all there. Good luck!