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

Ops_Biren 

As a newer, it’s really difficult to start. But in fact, database tuning is not a easy topic to describe shortly. If you would like to spend time to dig in it, please have a look for those links that replied by previous expert. 

Otherwise, I think I can give you some tips to get direction:

1. Clarify why does the tuning need to be done: ex: transactions work slowly, specific report runs slowly,...etc.

2. Gathering the data via SQL profiler and assigns the target database, tables...

3. Analyzing the data via SQL server optimizing tool: this tool can show you the suggestions for you. 
4. Choose the appropriate suggestion and then turn the target manually: Please DO NOT applies tuning suggestion via optimizing tool because you cannot realize what to be changed. 

The tips above are just give you a direction to start. Hope it is useful for you.