i doing SQL tuning in Company databases but i don't know where to start

Ops_Biren 

Try the steps provided here:

https://www.mssqltips.com/sqlservertutorial/276/sql-server-performance-tuning-and-monitoring-tutorial/

or here:

https://www.toptal.com/sql-server/how-to-tune-microsoft-sql-server-for-performance

To properly learn SQL tuning read all of the content here:
https://docs.microsoft.com/en-us/sql/relational-databases/performance/monitor-and-tune-for-performance?view=sql-server-ver15