Before you start planning, you must complete your primary research processes. These will help ensure you are not falling foul of any legal issues, not repeating the mistakes of competitors, and give your venture every chance of success.
Examine similar existing projects to ensure ...
Certain design formats have strong associations with particular products and services. Without making specific style decisions yet, determine whether your project should exhibit any pre-existing aesthetic approaches to accelerate end-user recognition of your aims and objectives.
For example ... a health related site will benefit from a clean, spacious, symmetrical and clinically precise interface and navigation. The same aesthetics would not suit a site which promotes anarchical heavy rock music.
Identify all the skills, knowledge and equipment you have available to you. You will want to steer your client towards using technologies and processes that are available to you, but also plan for circumstances where you must sub-contract, employ new staff and make new purchases.
What technology formats will be used for construction and devlivery? These will almost certainly depend on how the project will be viewed and expectations of what end-users have. eg If the project is primarily for iPad users there's no point using Flash.
Here are some possible technologies you might be using ...