I want to put one of those Keter Apex Shed in my backyard, it's
plastic.
it already has plastic floor with the package..

I would like to know what are my options on preping for a foundation
for the shed, which is going on a side of the backyard with currently
soil/dirt/weed.

I do not want to have someone come out and pour a concrete slab. Cost
is too high.

But I could like to lay some bricks, cinder block or concrete blocks.
I'm in California where the weather is mild, but I want to avoid
rotting or rodent problems.

Any advice is appeciated.