Without a photo it's a little difficult to be absolutely certain what your problem is, but I'll try to cover the ones that I think apply to your situation.
It sounds like you built your floor, then butted the walls on top of the floor, and any rain that catches the edge of the floor seeps under the wall. Regardless of which solution you choose, you should keep in mind that it's always a good idea to slope any exterior horizontal surface (even if only slightly) so it can shed water rather than allowing it to pool.
My first inclination is to say that you should have instead installed the wall frames out to the edge of the floor, then install your plywood to the outside so the bottom of the sheet extends below the top of the floor, and ideally below the bottom of the floor. This creates a drip edge so water follows the wall below the floor surface and the water cannot infiltrate into the structure. If your plywood does not extend past the bottom of the floor, you can install aluminum flashing to help carry the water below the floor.
The join is a horizontal plank joining a vertical one inn the middle.
So it's a floor joining to a wall.
Again, without a picture I can't be certain what your problem is, but this comment makes me think half of your floor is inside the structure and the other half is outside the structure, as in a porch. A possible solution in this case is to frame the wall and install right-angle flashing on the floor where the wall frame and floor meet. Caulk or otherwise seal it to the floor, then attach your plywood sheathing to the wall so it covers the vertical part of the flashing but does not extend all the way to the floor. This allows it to slide up and down with the seasons and prevents the bottom from absorbing pooled water on the floor and rotting.