Since aluminum is much more expensive than wood and not as versatile for construction, I recommend cedar wood framing for a beginning greenhouse. Please note that chemically treated lumber can be toxic to plants when water drips over it. Treated lumber can be used for side supports and exterior molding, but never for roof beams or areas above plants. Any drippage from treated lumber will be toxic to plants below. An aluminum storm door works well with an wooden frame and is more durable over time.
Location is important from a heat conservation and light availability perspective.
The south side obviously shelters the greenhouse from the cold northern winds and when the angle of light is lower on the horizon during winter, the south side captures more light during the shorter daylight period of winter. A greenhouse located in the middle of a yard without any barrier to the elements is the same as a fully northern exposed location. Although it would capture more light, it would be far less heat retentive. A location close to a circuit breaker box is best for accommodating electrical service to your greenhouse. Choose your site carefully.