Start the planning process with a good brainstorming sessions amongst your team members and if possible your client..  During this stage your client will likely have many ideas.

Once you have a goal in mind and an idea of who your target readers are it's time to get to grips with the site itself. Brainstorming is an excellent way to generate ideas. Grab
a large pad of paper and write down any ideas you can think of.

At this stage it's important not to worry about how silly an idea might be. You will be amazed at how a silly idea can lead
you into something new and exciting. Think of everything and anything that may be useful to the project no matter how far fetched it may seem. Get others to join in if you can. The more the merrier.

After a good brainstorming session you should have a pile of paper full of random ideas. It's time to comb through and sort out the solid ideas from the fluff. Consider each idea on its merits. You may find that an idea that seemed crazy at the time suddenly seems much more solid. Perhaps two or three random suggestions together make a terrific one.

When you've examined the results of your labours and come up with lots of good ideas you'll need to start sorting them into categories that will form the structure of your site.