Hello there!

First step is to point your domain on Godaddy.com to your droplet.

Here is a nice tutorial for that:
https://www.digitalocean.com/community/tutorials/how-to-point-to-digitalocean-nameservers-from-common-domain-registrars

Once done that you can just run your Flask app on port 80 and web requests will go right to it.

Best practices state that you should run a webserver that servers up the Flask app for you.
This way, if the flask app dies for some reason, you can have fall back page that will be served up by the webserver.

Here are some instructions to configure Nginx to act as your webserver (this will link to your Flask App).
https://www.digitalocean.com/community/tutorials/how-to-serve-flask-applications-with-uwsgi-and-nginx-on-ubuntu-16-04

Hope that helps!