Raised bed tips?

We’re about to build some beds for veggies. The beds will we be about 6ft long, 2ft wide and 2ft deep.

I’ve been researching a few different methods but wanted to see if y’all had any tried and true tips.

How deep does the soil actually need to be and what did you put at the bottom to fill up space?

Any recommendations on where we get our soil delivered from?

We’ve never done this before, so even if something seems obvious to you, please tell us!

Any mistake you made in the beginning that we can avoid?