There are lots of plans on the internet to look at and decide what you want it to look like.
I would make it out of cedar which is rot resistant. If you have a planer you could buy cedar fence boards and plane one side smooth for the top. Frame it with cedar 2X4s and on the bottom of the legs put something that will stop water from wicking into the wood if it sets on soil. Seal it with a clear deck sealer to help it resist water and stains.