A garden shed kit can be purchased that contains all the parts needed to build a shed. Parts come pre-cut and pre-drilled in the kit, so all that has to be done is to build it. Kits come with detailed instructions to help with the assembly.
This may be a good option if you have mid-level skills as a builder. Some feel that the kits sometimes lack quality when compared to a pre-built or custom-built shed.
What sometimes happens is that the buyer finds that they have taken on a project that is beyond their skill level. Then a professional will need to be brought in to finish the build. Unless you are very confident in your skill level, kits tend to be much more difficult than they appear.