new storage shed build

so theres an old kids fort on our property that wasnt getting used so i decided to to take one of the railings off the side so i could park tractors under it my plans are to put pea gravel down for the floor and build it as cheaply as i can anyone have advice on the thinnest plywood i can use as im trying to do this cheap all we have around the house is 2x4 4x4 and deck boards

Moved to "Off-Topic Project Builds"  

Plywood would expand pretty quickly especially when it rains.  Cheapest would be to tarp it. Unless you get a lot of wind then it may fray.  Long term cheaper would to just buy metal siding.  If wood, you will need to paint.  Same with plywood.

prolly will just put some pallets on the ground and leave it the way it is we will end up moving within a few years and its concreted down so i dont want to put money into it but if i can find a buttload of pallets ill use the slats to enclose it

You can get plastic roofing pretty cheap in various lengths and colors. Lasts decent and it's light weight. That would be a nice weather proof way to do it.

I used fence planks from privacy fencing. $1.50 for a 6 foot long by 5 1/2 inch wide plank that is pressure treated. Fence it in should be good enough to keep the weather off your parts. Which leaves the roof. I'd say pallets would be good to use but frustrating to take apart and get anything usable because the wood used is the worst wood of anything used for pallets.

I just used an outdoor grade wood paneling that came pre primed on my shed. That was about 5 years ago and I still have yet to actually paint it cause it's not getting messed with by the rain or snow here. It was pretty cost effective to.