For the foundation, you'll want to use concrete footings or a concrete pad, as that will be the sturdiest foundation. For the frame, you have several options, including douglas fir, pressure-treated pine, or metal. Douglas fir has beautful grain and is very sturdy, however it needs to be stained and sealed in order to prevent it from rotting. Pressure-treated pine is one of the most cost-effective and readily available materials, however it requires frequent staining to prevent cracking and weather damage. Metal can last for decades, but gets hotter during the summer. Lastly, you can use a metal roof for durability, or asphalt shingles for the roof if you want a more cost-effective option.