First download the rpm and install it. Then install the agent.
rpm -Uvh http://repo.zabbix.com/zabbix/3.4/rhel/6/x86_64/zabbix-release-3.4-1.el6.noarch.rpm
To install the agent, run
# yum install zabbix-agent
You can see the agent package installed and dependency packages also get installed.
Check the agent configuration file server IP and hostname of Zabbix server should be defined.
To start the agent service, run:
# service zabbix-agent start