Edit /etc/apt/sources.list.d/zabbix.list and paste the following lines (for 2.2)

deb http://repo.zabbix.com/zabbix/2.2/debian/ wheezy main contrib non-free
deb-src http://repo.zabbix.com/zabbix/2.2/debian/ wheezy main contrib non-free

or (for 2.4)

deb http://repo.zabbix.com/zabbix/2.2/debian/ wheezy main contrib non-free
deb-src http://repo.zabbix.com/zabbix/2.2/debian/ wheezy main contrib non-free

Then add the gpg key

gpg --keyserver keys.gnupg.net --recv D13D58E479EA5ED4
gpg --export --armor D13D58E479EA5ED4 | sudo apt-key add -

and update the apt package list with

apt-get update

Install the Zabbix agent

apt-get install zabbix-agent

and edit /etc/zabbix/zabbix_agentd.conf to configure it. In a simple passive-agent configuration you have at least to set the server address

Server=my.server.address

Finally, restart the agent to reload configuration with

service zabbix-agent restart