Don't know where to start.... Prep work?

[WB72]

I just removed the box from the old girl,(get your mind out of the gutter)and want to know all there is to know about prepping for paint. It's a wooden box, It needs a couple of patches, but mostly surface rust(I think).

Do I start with the welding or taking it to bare metal or take the floor out. I do want to re-do the floor, what material is good for the floor?

I need every bit of info there is. What grit paper to use, I am a Virgin (there you go again) with body work. I will take all tips greatiously.