How do I improve my Vocals?

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People tell me I have a good voice when I sing.I think it's okay but not great.I think it could use way more improvement.Everyone says it's soft and sweet,that it sounds younger than how old I really I am.So how do I improve my vocals so I Can sound stronger...7 years Choir and 2 years Music class and I still have my vocals at where they were.My instructor says I have a lot of potential but I'm just not using it all...but I don't know how to push my self past my limits,what should I do? :/