how to use custom visualforce component into a visualforce page in Salesforce?

Hi,

To use a custom component in a Visualforce page you must prefix the component’s name with the namespace in which the component was defined. For example, if a component named myComponent is defined in a namespace called myNS , the component can be referenced in a Visualforce page as <myNS:myComponent> .

Please follow the below link for more details-

https://developer.salesforce.com/docs/atlas.en-us.pages.meta/pages/pages_compref_component.htm

Thanks.