I have a singing question?

I am 14 years old and a soprano. I can reach up to a high d (2 octaves above middle c), sometimes higher, sometimes lower. I can reach down to about an e one octave below middle c. I want a wider range, and it has increased drastically over the past year, but I don't know whether to focus on going lower or higher. I want to be in musical theater someday. My idols for singing are people like Kristen Chenoweth, Madeline Kahn, and Julie Andrews. I want to have around a 4 octave range when i grow up! How can I achive this? And also on which note does the whistle register start?