Visit your local library. In the design, home improvement, decorating or construction sections, look for books ranging from "best-of" do-it-yourself projects to full plans with measurements and step-by-step instructions. You may also find inspiration by checking out books on interior design or decorating.
Search online. Websites like Dornob feature many designs ranging from simple modifications of existing furniture to complex pieces made out of tree trunk or vinyl. Instructables.com has hundreds of tutorials to teach you how to build shelves, as well as many other items. Many of the designs are "eco-friendly" and made from materials such as reclaimed wood and recycled beer bottles.
Explore local resources like workshops, guilds or crafting hobbyists. These workshops offer a chance to meet like-minded hobbyists. Another alternative might be to coordinate a group in your area.