SQL Server 2016- Email Alerts fo Errors/ Warnings
Your question is very broad.  I hope you have some understanding or clarity on how SQL Server alerts/warnings work and what are the things you need to monitor based on your business need. 

As a start read this about 3 types of alert that you can configure via SQL Server Agent.

Alerts

There are numerous articles written online about this topic and some scripts are available that you can use once you understand what you need.  

If you have replication set up you will see a bunch of predefined alert show up in your SQL Server agent under alerts. Read details below:

Configure Predefined Replication Alerts (SQL Server Management Studio)

There will always be situation that you need to be alerted that is not covered above and you have to roll out your own scripts.  That can be using tsql, Powershell and run via SQL Agent Jobs.  If you manage many SQL Server instances you might have consider having a central monitoring/DBA instance and manage alerts from there. 

Few resource I am listing for you to start with:


SQL Server Alerts: Soup to Nuts by Robyn Page and Phil Factor
SQL Server Agent Alerts by Tim Radney
Blitz Result: No SQL Server Agent Alerts Configured 
The Ten Commandments of SQL Server Monitoring by Adam Machanic