The first thing you need to do is to make sure you have an MX record for the domain (or for each domain) you want your new mail server to serve.
To do that, log into your account and click on DNS from the list of links on the left of your screen. Then click on the little magnifying glass icon you want to set the record for, or do that for each domain, one at a time. The “Create New Record” tab offers a list in a row from which you select MX.
For each step, there is a question mark at the top right of your screen and clicking on it will offer links to helpful tips.
That is the easy bit. Installing and configuring a mail server is a complex process and I would classify it as “difficult” for anybody inexperienced. I haven’t found a tutorial for a simple mail server on DigitalOcean so I would do a web search. Learn as much as you can before you start. You may even like to rent another droplet and point a subdomain at it to practice on.
Warning: if you don’t know how to set up DNS records for MX and A records you really do need a lot of time to put a mail server together. Another tutorial you may like to have a look at - click on the link - tells you why not to run your own mail server.
Another place you could start is with this tutorial - click on the link. But this is quite a complex example and is asking an awful lot of an inexperienced sysadmin.