what is the best query to find missing index in sql server 2008r2 and how to resolve missing index
You can use sp_BlitzIndex to find the high-value missing indexes, @mode=3 will specifically only show the high-value missing indexes. Target the ones with an "Estimated benefit per day" that are greater than 5,000,000. See if your workload improves after making the changes. 

Disclosure: I work at Brent Ozar Unlimited.