Whew! Huge question, Guest! It needs a week's answer, or none. you may want to look it up in GoYpedia first, then ask what you need to after.

Cheat buy link-o boards these are the plastic oblong boxes to grow your veggies in you just add soil. Simple. Look at my photos and you'll see what I mean.

Great thought, Great! Plastic would probably last longer than wood in the UK--ulike here in the desert--and those are quick and easy to assemble. Long term, you may want to gradually replace them with masonry of some sort, as you gain experience, and finalize your garden plans. Those stackable, rough-surfaced, interlocking cinder blocks can make a relatively easy alternative to a full-up ashlar wall, or hiring a skilled dry waller. Soil mixes are probably better addressed by someone in your area, because our soil and weather force weird compromises on us Phoenicians.