Things I have learnt over the years.
Supermarkets often have very cheap plants, as above post says.
Chain Garden centres seem to me to be more expensive and just buy in stuff when it’s actually flowering, so by the time you get it in the ground it’s past it’s best.
Nurseries are generally where you will get more perennials (they last the winter and flower again next year). More expertise and not necessarily cheap, but should be value for money.
I have been gardening for many years, but still don’t feel I know much.
My advice, for every plant you research and buy carefully, allow yourself to buy something cheap and cheerful on a whim. Sometimes we learn Best through trial and error.
Bedding plants like geraniums, lobelia and petunias are great to buy at the beginning of summer (they only last the summer), for instant colour. (I mostly put them in pots.).
It’s late summer now, so not worth buying any bedding stuff.
Remember it should be fun and relaxing. The trend for natural gardens good for wildlife is big and allows for more hodgepodge styles rather than the regimented colour coordinated perfect style of yesteryear.
Please don’t get fooled into thinking you need bug and weed sprays. Evolution has given every greenfly and slug a place on the food chain. If you kill something at the bottom it affects something at the top. We have less birds, hedgehogs etc than ever.