See https://www.sqlshack.com/sql-server-performance-tuning-tips-for-beginners/
I have not read the article myself, but it was the top hit when I entered introduction to sql server performance tuning in the search box Google. There may be want to more hits that you want to pursue.
If you have a specific problem you need help with, you are welcome to ask, but you need to share enough details so that we can help.