Before creating any website its common practice to visualize the layout, the design and all the features you intend to incorporate. In addition, you think about how users will interact with each page and how the site should perform (behavior, load time etc.). So, establishing a list of all functional and non functional requirements is essential and first step of development.
Project planning involves clarifying an objective (project, idea, concept etc.) by proper documentation of characteristics, requirements, dimensions, duration estimates, cost estimates, timelines and resources to an acceptable extent for reference purposes. The total cost of a project with proper planning is less expensive compared to having no plans, and it puts the power to control a project to successful completion in the hands of the project manager and his team.
Mistakes made at the planning phase are far less expensive than those made during execution. If you do not want to make expensive mistakes, please do all you can to give attention to planning, planning and planning as this will put control power in your hands.