If nature hasn't provided you with the ideal location, all you need is a few loads of topsoil and some rocks. If you must bring in your own rocks and soil, start small. The job may be bigger than you think. Rocks Rocks native to your area will look natural and will be the cheapest and easiest to obtain. Large rocks with irregular shapes look interesting in the rock garden, but keep in mind that you'll need smaller rocks, too.
Limestone is a good selection.
The rock is soft and porous, allowing moisture to seep through, and some determined roots will be able to make their way through the rock. Limestone usually has depressions in it that can be filled with dirt for planting mosses and lichens. Set the rocks in the lowest, front part of the garden first and work upward. Shovel enough soil around each rock to anchor it firmly. You may need to bury half or more of each rock.