The best supplies for hiking?

I've been hiking in the past but nothing too intense. I didn't buy a pair of boots until last year and they aren't very good for an entire day of hiking. I've joined a hiking group and went on a 20+ km hike for the first time to realise my boots were not cut out for the job. In the past, I've just been in runners which was fine as I hadn't been on any particularly mucky trails. Unfortunately I'm in the UK now and 'particularly mucky trails' are the norm.

What kind of supplies should I have for day hikes? And what are the best versions of those supplies? And where can I order them? My town doesn't really have outdoor stores so online ordering would be best or stores in London.

I'm also working up to multi day hikes so wanted to know what would be best for those as well.

I'm a complete beginner for all intents and purposes, and need some guidance beyond what Google can offer.

Thank you!