If you have the right climate, your back deck can almost be as good as an additional room on the house, and is certainly cheaper than adding on to the house! Most people choose between wood and plastic furniture.
Wood is typically more attractive and classic, but is more expensive, weighs a bit more, and may be a bit more difficult to maintain over time.
Plastic may not look quite as great, but is typically not expensive to buy, is very lightweight, and will last a long time, even when exposed to the elements like rain, snow and constant sunlight.
Both have advantages and disadvantages, and will allow you to spend more time in the backyard.
If you're not sure what to get, one smart idea is to start out with a single chair and a sidetable.
This way, you can see if you actually like spending time sitting on the deck without investing in a full set of four chairs and a large table.
Once you confirm that you like sitting outdoors, buy the whole set with the confidence that you'll get years of great use from it!