To import your Ancestry tree into Legacy, you need to create a gedcom on Ancestry.com. Only the owner of the tree can create the gedcom so if you're not the owner, you'll need to contact that person and ask them if they would create a gedcom for you.
Detailed instructions with images are on the Ancestry website at
Once a GEDCOM file has been exported from Ancestry.com, you can easily import it into Legacy by clicking on File and selecting Import From > GEDCOM File and follow the prompts. Please Click Here for more details on how to import a GEDCOM in Legacy.