You can follow the documentation for instructions on Aggregate installation: https://docs.opendatakit.org/aggregate-setup/
However, I would suggest you explore Central. This forum post discusses the features currently available. We expect Central to become a viable replacement for Aggregate for most users by June 2020, if not sooner! I think the instructions for setting up ODK Central on Digital Ocean are much easier to accomplish than setting up Aggregate: https://docs.opendatakit.org/central-setup/