Well, this is a rather open-ended question.
Basically, you need to
- Install ZenOSS on a server
- Make it discover the devices it should monitor (you may need to install/activate SNMP or other services on the devices, so they provide monitoring information)
- Configure what it should monitor on the device(s)
- define rules about who should be alerted when there are problems (if you want that)
The details will depend very much on your setup and your needs.
To get you started, first read and follow the Installation guide. Then work through the Administration Guide, which will help you set up basic monitoring.
By then you should have most basic functionality. Then it depends on your needs. Browse the available Zenoss docs, and try things out. Then if you are having trouble, you can ask a specific question here.
Have fun with Zenoss, it's a cool system!