Start with a budget. Shop with a budget in mind so that you don't waste time evaluating furniture you can't afford. A budget also helps you trim the list of furniture vendors you should visit.
Define the function of the patio and garden furniture. If it's for dining, how big should the table be? How many chairs will you need? Patio furniture could also be for lounging and quiet reflection (hammocks, lounge chairs with low side tables or gliders with ottomans), chatting (single chairs with a center coffee table), viewing (swings or rocking chairs) or casual seating (benches and utility tables). Should they be light enough to move around or take indoors?
Consider the location of the patio or garden. Is it out in the open, covered or screened? When you choose your furniture, consider the type of exposure it will have. Sunlight will make cushion colors fade and some synthetic materials deteriorate quickly. Genuine wicker will not be able to withstand constant wet and humid weather. The furniture you choose should be able to withstand whatever weather conditions it will be subjected to.
Determine the style and décor theme of the place. Victorian flair? Country cottage? Asian-inspired? Contemporary chic? Coordinated décor and furniture helps bring the whole place together.
Check for comfort. Outdoor furniture is usually meant for relaxing, fun moments with family and friends. Be sure you will be comfortable for extended periods of time on those tables and chairs. Remember that bare metal can get too hot or too cold. Also, some pieces can look good, but don't contour well to the body. Always try the furniture before committing to buy.
Think about the furniture's maintenance requirements. Because it is exposed to the forces of nature, outdoor furniture needs to be regularly cleaned and maintained. Plastic and aluminum pieces are low maintenance and easy to clean (soapy water and a strong hose). Metal furniture needs to be checked and refurbished to fight corrosion. Wood tables and chairs need to be resealed or repainted. Cushions need to be replaced or repaired as needed. Consider having to deal with these tasks for the life of the furniture.