Scrum questions

1. How to help a product owner to prioritize user stories if he/she is new in the role from scrum master point of view

Start on the basis that the Product Owner orders the Product Backlog, and that this might involve prioritization. There are other things a Product Owner should consider as well...what do you think they might be?

2. What are the different metrics/techniques which can be followed in Kanban and Scrum related to user stories

Remember that a user story means having a conversation about a potential requirement and that value is what must be ultimately accounted for.

1. My first opinion is that you don't prioritize, you order.  Priority is just one piece of the information that is useful.  There is also dependency, revenue opportunity to name a few.  Consider the example that your company is trying to get a new product to market and that effort is thought be 5 months of effort. That could be considered the highest priority.  But you might have a product enhancement that will be able to net you 1000 new clients and that work is only thought to be 4 weeks of effort. Given the increase in revenue and the short expected time frame the feature enhancement might be ordered above the highest priority. 

This is not about prioritizing as much as ordering.  Remember this statement from the Scrum Guide

The Product Owner is accountable for maximizing the value of the product resulting from the work of the Scrum Team

2. What are the metrics that measure what you deem most important? 

I'm going to suggest some that I use in both Scrum and Kanban.  Cycle time, Lead time, Throughput, Work in Progress (WIP).  They all are based upon the Product Backlog Items (Scrum) or the cards that represent the value opportunity (Kanban).  For me any other metric would be used if the Scrum Team felt it would benefit them in doing their work.  Many of those are not related to items on the board.