Create a back up of everything on your existing site and store it someplace safe.
Make a list of everyone who has been involved in creating and maintaining your website. Are these people still active in maintaining your site? They should be included in this process as well, or at the very least, told that the migration is about to take place.
If it’s just you, you might want to enlist the help of other stakeholders in your group. More heads are better than one for being creative and finding out what your users need.