Agents are deployed on the target servers which are being monitored from 12c cloud control . Use below command to start the agent.

Checking the agent status:
oracle@test:…oracle/agent12c/agent_inst/bin$. /emctl status agent

Oracle Enterprise Manager Cloud Control 12c Release 5
Copyright (c) 1996, 2015 Oracle Corporation.	All rights reserved.

---------------------------------------------------------------
Agent is Not Running

Starting the Agent
oracle@test:…oracle/agent12c/agent_inst/bin$ ./emctl start agent

Oracle Enterprise Manager Cloud Control 12c Release 5

Copyright (c) 1996, 2015 Oracle Corporation.	All rights reserved.
Starting agent .............................. started.


Check the status:
oracle@test:…oracle/agent12c/agent_inst/bin$ ./emctl status agent

Oracle Enterprise Manager Cloud Control 12c Release 5
Copyright (c) 1996, 2015	Oracle Corporation.  All rights reserved.
---------------------------------------------------------------
Agent Version	: 12.1.0.5.0
OMS Version	: 12.1.0.5.0
Protocol Version	: 12.1.0.1.0
Agent Home	: /oracle/agent12c/agent_inst
Agent Log Directory	: /oracle/agent12c/agent_inst/sysman/log
Agent Binaries	: /oracle/agent12c/core/12.1.0.5.0
Agent Process ID	:	11952
Parent Process ID	:	11938
Agent URL	:	https://192.20.204.64:3872/emd/main/
Local Agent URL in NAT	:	https://192.20.204.64:3872/emd/main/
Repository URL	:	https://test.test.com.sa:1159/empbs/upload
Started at	:	2015-11-19 09:46:09
Started by user	:	oracle
Operating System	:	SunOS version 5.11 (sparcv9)
Last Reload	:	(none)

Last successful upload	:	2015-11-19 09:46:30
Last attempted upload	:	2015 -11-19 09:46:30
Total Megabytes of XML files uploaded so far :	0.02
Number of XML files pending upload	:	0
Size of XML files pending upload(MB)	:	0
Available disk space on upload filesystem	: 57.38%
Collection Status	: Collections enabled
Heartbeat Status	: Ok	
Last attempted heartbeat to OMS	:	2015 -11-19	09:46:27
Last successful heartbeat to OMS	: 2015-11-19	09:46:27
Next scheduled heartbeat to OMS	:	2015 -11-19	09:47:28

---------------------------------------------------------------
Agent is Running and Ready

To stop the agent use :

emctl stop agent