How can I visit the great wall from Kunming? Can I go by Metro?

No, you cannot visit the great wall from this city by metro.
The Great Wall is in Beijing. You need to take a train to West Beijing Railway Station first.
About the great wall, do you refer to Badaling Great Wall?
If yes, you can bus no. Special 13 to Deshengmen Station. Then take bus no. 919 to Badaling Station directly. After arriving, just walk to the scenic area.

Thank you, what would be the cost & time?

China Great Wall has many sections and the Badaling and Mutianyu around Beijing are the most famous. And Badaling welcomes the most visitors.

The train between KM and Beijing takes almost 36h. I would recommend you fly to Beijing in around 3h and 20min. The air ticket fare can be found on this website. After that, take airport shuttle bus line 2 to Xizhimen Qiaonan at the cost of CNY24. Walk from the dropping-off site to Beijing North Railway Station to take a train to Badaling directly in around 70min and the train costs CNY6 for a hard seat.