For the best results, make sure the container you use has holes punched in the bottom. The more the better. Fill it with potting soil mix. Don't use "plain dirt" from the garden. Plants will not be happy in it and probably not grow as well.
The soil needs to be mixed with other things so water can get out and the roots have enough air. Use one shovel of garden soil, one shovel of peat moss, and one shovel of perlite (white gravelly looking stuff) or very coarse sand. Mix it together and you're ready. If you want to buy soil, get something like potting soil mix that has all the good stuff in it already.
Don't forget to water regularly because these special place gardens sometimes dry out very fast. Put enough water on top of the soil so it starts to leak out the bottom. Also, don't forget to fertilize. Use a liquid fertilizer about every two weeks. If you don't, the plants will get hungry and not do as well. Also, there are no rules about what you can grow. Grow what you like and have fun.
So now that you know the basics, check out these unusual places and spaces. And don't forget these are only ideas. With a little imagination you can come up with an unusual place and space of your own. If you do, please send us a picture. We would like to see it and others may want to try it out.