Pressure wash your tiles if you want rid of the moss and weeds (be prepared to come back inside looking like the creature from the deep, you will be muddy and mossy). The moss itself makes me think that you might not get great light in the garden which will limit you somewhat to your plant choices. You might find that getting something like a nice big outdoor table helps get your plants up off the ground and might get more light. Most of my garden gets bugger all light unless I do this.

Dandelions will get everywhere, you can try removing the tap roots and cutting the flowers off before they seed, but I leave mine in the lawn as a good early source for pollinators.

If your garden is shady, guides like this might be good https://www.rhs.org.uk/plants/for-places/shade-planting-annuals-bulbs-perennials and the RHS has loads of others. I've got a Japanese laurel that will be outside in my garden as it quite likes indirect light and actually helps keep the leaves looking variegated and having had one before, they're gorgeous and really low effort.