Best practices for ensuring best start possible on new design project?

I am working on redeveloping a website from scratch from an existing decade old legacy site. I am having a difficult time understanding the subject and objectives of the new site. What is the recommended process for ensuring we get started on the right path for the new site? Do we start with a client questionnaire? Any good examples out there? Secondly, at what point does a flow diagram enter the pre-production planning stages? Any tips for ensuring a great start would be highly appreciative!