In Resource Monitor you can move between the various tabs: Overview, CPU, Memory, Disk, and Network.
The selected tab will present one or more areas listing process, services, and applications that are running.
You can click the column headers to do some simple sorts: high to low or vice versa.
What you are looking for is some process, service or application that is using a noticeably high % of one of your system's resources.
Enlarge the window so you can watch as much as you can without the need to scroll or move the mouse.
You might find it easier to explore Task Manager first. Learn from that and then take another look at Resource Monitor.
No need to immediately do anything except look and observe. Do keep in mind that you should allow your system some time to stabilize: meaning you do not do anything other that open the applicable tool and wait until the system is likewise finished with any immediate startup taskings. E.g., backups, scans, updates.