Before you start using your own domains you should set up their A-records to the corresponding IP-addresses of virtual servers you've created on Digital Ocean. Without this set up you won't be able to use your domains.
This basic step by step guide can be applied for almost all domain register sites. Do the following:
You need to wait up to 4 hours (usually much less) for DNS being updated.
Here’s the example on how to do this in Namecheap.com