A detailed explanation of zabbix server and client installation and its concepts are mentioned in the below links.
Zabbix Installation
Basic Concepts
Here I will explain how to monitor a website, Its downloading speed and successful logins.
For web monitoring we need to,
Create a Host from the configuration tab.
create an application for that host and name it as web_sitename or whatever.
Then go to "web" from configuration tab, From there you can create new scenario.
We need to specify,
1. application name
2. Scenario name
3. Update interval
4. Agent (any listed browser)
5. variable ( Can give username and password as variables )
For example {user}=admin
{password}=zabbix