sp_sysmon requires you to set enable monitoring to true (enabled), and that the user have the mon_role:
sp_configure 'enable monitoring', 1 grant role mon_role to sa
Use sp_sysmon with:
A fixed time interval to provide a sample for a specified number of minutes
The begin_sample and end_sample parameters to start and stop sampling
Tailor the output to provide the information you need. You can:
Print the entire report.
Print only one section of the report, such as “Cache Management” or “Lock Management.”
Include application-level detailed reporting for named applications (such as isql, bcp, or any named application) and for combinations of named applications and user names. The default is to omit this section.