I've been practicing for a long time. Ever since I was a little girl. I can get low notes done perfectly. But, I want to get the more higher notes~ I can't seem to be able to do it. Maybe I still have a kid-ish voice because I'm still a teen, so I can't get it right? Ehh, any tips at all? Thanks in advanced. <3
Use less air pressure for higher notes and relax your voice. Don't try and hit the high notes. Just sing lyrics clearly and let your particular vocal type hit the notes it is naturally able to hit. You'll generally hit those high notes you want to hit when you strive for this. Also don't try and sing for a vocal type that is different than your own, that plays into letting your voice naturally hit the notes it can. You have to use your instrument the way it is meant to be used. So if you're a Mezzo-Soprano, you don't want to be trying to sing Alto or Soprano. Sing Mezzo because that is what your instrument is and don't let anybody make you sing for a vocal type that isn't your own. 
Use less air pressure for higher notes and relax your voice. Don't try and hit the high notes. Just sing lyrics clearly and let your particular vocal type hit the notes it is naturally able to hit. You'll generally hit those high notes you want to hit when you strive for this. Also don't try and sing for a vocal type that is different than your own, that plays into letting your voice naturally hit the notes it can. You have to use your instrument the way it is meant to be used. So if you're a Mezzo-Soprano, you don't want to be trying to sing Alto or Soprano. Sing Mezzo because that is what your instrument is and don't let anybody make you sing for a vocal type that isn't your own.
References :
me to.
its super simple
to sum it all up.
sing louder and sing higher.
even if it doesn't sound like mariah carey
it will help.
now i can sing really high.
i hope that helps
<3
References :
personal experience
It would be a good idea to approach a higher note with a softer and quieter voice. After a while you can work yourself up to a louder volume. It's trying to relax the muscles in your throat yet make them stronger at the same time that way you have a stronger head voice.
References :