Z weather strip flashing is a great way to protect the horizontal edges when installing siding on your shed. Typically the Z strip is placed on top of the bottom piece of siding, then the next row of siding is placed on top of the Z strip.
This weather strip comes in plastic or galvanized metal. I prefer to use the metal version of the z strip because it will not crack and deteriorate like the plastic. What I use is the 1/2" because it fits perfectly when building a shed with 1/2" siding panels.
You can usually find this flashing in the section of your building supply store where the drip edge and rain gutters are located. It normally comes in 10' lengths.