How do I build an easy foundation for a shed?

Here's a simple one-https://www.youtube.com/watch?v=vDRrmHMaQ1s

Cheap shipping pallets, that's what I used works great.

How long your shed lasts will depend on how good the foundation is... especially if the shed is plastic... a shifting foundation will effect how the roof fits, and how the doors close.... spend the funds and take the time now to give it a good foundation.. and it will hold up longer.... and so will your temper.

I am assuming that your location does not require a solid base such as concrete. Where I live sheds have to be on a concrete pad that is at least 6 inches thick. If you don't have to do that I suggest pallets. They are easy to work with, you can get them for free (a plus!) and you can stain or paint them if you want to. If you need a floor that you can walk easily on cover the pallets with plywood.

Paving stones, bricks or railroad ties butted together.

Just be aware that most bases made out of wood rot unless they are treated.

if you use pallets or other untreated wood it will rot eventually, at that point you will be sorry that you didn't do it right the first time (trust me, I've made that mistake too many times). I did this for my grandson and my suggestion is to make a base of pressure treated 2 x 4s, with a plywood cap. It will be more work but will last for years and years. I've learned that it is always better in the long run to do it right the first time.

We put a shed in a sand box that once contained a swing set. We leveled pallets it the sand and put the shed on top.