Singing..help?

im 14 yrs old and i love singing,,im good cept for when i try to go any bit high...i can only sing low....i sing the song As I am by miley cyrus pretty good...dont diss her please...she is my fave singer ever!!!!! anyways, i am trying to improve my vocal range so i can sing more....im an alto in school too...does anyone have any tips? also, i am shy with my voice so i dont take lessons i only let my mom and a couple of my friends hear me..any tips with that too??? i appreceate it.. and please dont say mean stuff about me or hannah montana....and dont say i dont want to sing like her because she sucks or anything or i will report you...thanks!!