New to online grocery shopping - advice needed!

[Penn2021]

I’ve never needed to do online grocery shopping before (tiny family, near a large store, love seeing the goods!). I buy most of everything else on line. I suddenly find myself needing to shop online as I need to give a family member end of life care and won’t be able to leave the house and go to the stores.
Can I ask for some advice please? Which stores are most “reliable”? Stores reasonably near to here are Tesco, JS, Morrison’s (can get discount with them), Waitrose. I can’t seem to find any slots for the next 3 weeks in any stores locally. Is there a particular time they release new slots that I should be lurking for?
Any general tips or advice on things I should be doing or not doing.
Many thanks!

I've found it nigh on impossible to get a delivery slot since March, annoying as I have shopped online for over 20 years, and never set foot in a supermarket.

I now do click and collect. In your situation, I would consider stocking up for a few weeks on your frozen, long life, tinned and dry goods. Milk and More were useful with some fresh items, leaving fresh fruit and veg. Iceland have been OK with this as has the overpriced local Co op.

The very best to you and your mum.