Hi @aravind1patil1 

Welcome to ServiceNow community man, glad you're learning about Servicenow

for Catalog Item you can watch the below lectures

And your question title, you talked about Catalog item in service portal, 

see the functionality of catalog item remains same whether it be Native UI or Service Portal, But the thing is in Service Portal you have Widgets,Pages, also sometimes we do implement some functionality for service portal, so for that you can learn service portal, below I'm attaching a good tutorial

If my Solution helped, Please Mark it helpful & 

Regards

Sumit Y