How can I learn to sing better?

I'm a good singer. But, I'm not amazing. I'm not very good at hitting high notes, how can I get better? I want to get better at singing, but I can never practice becuase I have no where to practice. I don't like to sing when my family is around because I get self consious. But, I do want to sing in my school musical. Chorus, though, starts midway in the year. Any, help, ideas, tips, tricks would be gladly appreciated.