Beans are a practical and healthy addition to any garden, but they grow best when they have something to climb. Sure, you could buy a trellis, but it's extremely cheap to build your own.
All you really need are some long poles and twine. Michelle Reynolds at Made+remade suggests bamboo, but long dowel rods, or even PVC, would probably work just as well (though they wouldn't look as nice). Basically, you need to tie together a few tripod-like structures with the rods, and hang two additional rods across the top and bottom of the installation. Once everything is secure and firmly planted in your garden, run a single long strand of twine in a zig-zag pattern across the entire structure.
Once you're done, wait for your bean plants to sprout, and then manipulate them onto the loops of twine to "train" them to climb. In no time, the whole structure should be filled with the edible products of your own backyard garden. For some great pictures and a step-by-step guide, be sure to click through the source link.
Make a Simple Bean Trellis for Your Garden [Made+remade]