what monitor before applying changes on the server? - looking for a basic answer - starting point only
I believe starting with Baseline Data would be good to monitor before and after changes.

Information which you might want to capture like:


Basic Information
System Usage
File and Database Sizing Information
Wait Statistics   etc..


If you do not want to use third party tools, you should start looking into collecting above data and when as explained 5 Reasons You Must Start Capturing Baseline Data

Also, in order to set these baselines, you will need to create a database and can capture for duration according to the workload behaviors as explained in
Capturing Baselines on Production SQL Servers.

Also refer to this MSDN forum discussion on same.