The first piece of advice is ignore everything a shop tells you. You would think they should know but so many workers haven't a clue and make things up to get a sale.
Always do your own research. Ask on here or look at fish on
https://www.seriouslyfish.com/knowledge-base/ Anyone can set up a website and sound like an expert, but this site
is written by experts.
Most tanks in the UK come as a complete set with the tank, lighting, filter and heater. Get the largest that will fit and you can afford.
Think about whether you want live plants or fake ones. It makes a difference to the way a tank is prepared for fish.
If you choose fake plants, you need to do a fishless cycle the tank before you can get fish. If you choose live plants, and more than just the odd one or two, the tank is cycled differently. Rather than throw a lot of information at you now, we can discuss these once you have bought a tank.
Look on your water company's website for hardness. If they give it, you need a number and the unit of measurement (there are several different units) rather than some vague words. Hardness in important for fish and we should aim to keep fish which come from water with a similar hardness to our tap water.
Finally, my first piece of advice again. Don't listen to what the shop says.