Somewhere in Ancestry, you can download your tree as a GEDCOM file. That file can then be uploaded to WikiTree to get you started.
Personally, I have not tried this. When I switched from Ancestry to WikiTree, I created new profiles so I could make sure I controlled the information I was providing and not relying on the numerous dubious sources I ended up having in my Ancestry tree. Of course, that might not be practical if you already have a large tree so the GEDCOM method might be best for you.