Visit the Claim Teams page:
https://ottoneu.fangraphs.com/claimteams
And click “Claim” on the right of the team you’re interested in claiming. If the league is private, you will need the password from the commissioner, which you can get by reaching out directly to them (League Settings -> Commissioners -> User Profile) or by posting here in the Owners Wanted forum. You can also browse the Owners Wanted forum and post in any topics that from leagues of interest.