The answer to your question depends on the web designer. Some of them will want a lot; others will want less. Depends on what they've agreed to do for you, what they feel comfortable handling and what your agreement specifies you will do or provide for them.
The first thing you need to do is decide what exactly you want your website to do (not how you want it to look -- what you want it to do
). And by that I mean, what goal(s) you want your site to accomplish, not what fancy bells and whistles you want on your pages. Do you want to make direct sales? Generate leads for yourself or an affiliate site? Get people to sign up for a newsletter?
Once you can clearly articulate exactly what you want your site to accomplish, find a designer/developer that can help you reach that goal. They should be someone you're comfortable working with, someone you feel has the expertise and skill to do what needs to be done with your site. They will be able to guide you as to what page layout, site architecture and features will best serve your goals. Listen to their advice. You're going to hire them because they're the expert, the professional. Allow them to do their job.
Then negotiate with them about what you're going to provide and what they're going to provide. If they've done this before they should have some sort of checklist or agreement or something that can give you an idea of what you'll need to furnish for them.
No point in driving yourself crazy trying to think of everything before the fact. Find a good designer and let them help guide you.