Building a garden deck by yourself can be a very labour intensive job. Like with any structure, making sure that its foundations are suitable is the first, and most important step. Dirt makes a poor foundation, even for something as relatively light as a garden deck. In order to shore up the foundation of your deck, begin by digging out a sizeable hole underneath. Once this is done, you should then begin by placing gravel along the base, to give a solid base to it. From there, the best, most cost effective option is to use builders sandbags, which will give you a solid basis on which to develop the rest of the construction.
A garden deck can be made with a variety of materials, ranging from various types of wood, to different types of natural, or unnatural rock (brick). Using metal over the top of the wood to help to protect the materials. Using wood, while in my opinion the best looking type of deck, can expose your construction to decay. This is a bad thing in the best of circumstances, but if parts of the deck are hollow, it can also be quite dangerous.