If you haven't much money then as others have said lots of water and free exercise walking/running/yoga with Adrienne on Youtube.
Boots do good deals on body care/face cream/make/hair care up on £10 Tuesday. Ask people for vouchers at Christmas and use cashback sites like Quidco to maximise your spend and perhaps get discounts.
If you don't have lots of money for clothes then get a basic capsule wardrobe going, choose a base colour black/navy/grey then build around that. Primark do pretty undies quite cheap and Sainsbury's TU range and sometimes knowing you have something super feminine on underneath can be a real boost.
Look after small details yourself, you can do a mani/pedi at home for example. A neutral pink on the hands looks elegant and wears better than bright colours. You can go a bit more colourful on your feet, like a classic red.
Buy interdental brushes and USE them as teeth are one issue but if your gums aren't good, it can make teeth worse - your dentist can advise on sizes.
There are nice free meditation videos on youtube to help with confidence and also tons of advice from women on looking elegant etc on a budget. Dress for your body shape too to make the most of your good bits and we all have them - boobs, shoulders, legs etc to draw attention to them. There is a woman called Anna Bey who basically hauled herself up by her bootstraps and has developed a refined look and coaches on elegant outfits, manners etc but there are lots of them online.
Definitely save up (or ask for money at Christmas) for a good hair cut and consultation as that will make a big difference. You don't have to dye your hair in a salon. You can buy good at home hair colour (semi-permanent is good to try out for a sheen of colour) now that is multi-tonal as long as you stay within two shades of your own colour - Superdrug often do good offers and their own range hair colours are nice.
You could pop into a nice department store and go to a posh makeup counter, pretend you are interested in their products and get them to help you look at make up colours/techniques that suit you. make a note then find drugstore dupes (lots of websites about this) to buy so you can recreate the looks. You can also grab some samples as well!