How can I be a better badminton player?

How can I be a better badminton player? Sometimes, I can play as well as my older sis, but sometimes I can't...I surfed a lot of websites but it didn't help at all! I also tried to get a coach but it's really expensive... So I was wondering, is it me or is it my badminton racket? Coz my racket isn't branded and it's really cheap...