While your question is quite generic, I will try to address it to cover most.
I want to ask VMware ESXi HOST and Guest based on which metric you must follow threshold.
Well, if you wish to monitor the CPU, Memory, disk space/writes, network traffic, you can always use:
1. esxtop http://communities.vmware.com/docs/DOC-9279
2. VEEAM or SolarWinds
3. Any additional software http://communities.vmware.com/thread/137349
How do you monitor VMware. What should I do.
If you are mentioning about the server hardware health, then you can always use the management software based on your server.
