Kendra Little gives some tips on how to gather performance tuning skills:
Start writing queries that demonstrate TSQL anti patterns – and make them slow
You know how people say that the best way to learn something is to teach it?
The best way to learn to speed up queries is to write slow ones.
The best way to get a job speeding up queries is to write a blog about the queries you’ve sped up.
This is a long-term learning process, but is absolutely a worthwhile skill for any database professional.