#1
I think my voice is so childish, and I want to sing like Liam Gallagher any tips for improving my voice?

Ps.: I can sing Wonderwall almost like Liam , but when I try to sing anything else my voice seems to be from a 10 years kid.
#3
A mature sounding voice is all about resonance and technique. Try singing "from the back of your head" instead of pushing the sound through your throat. Your going to try to loosen up your throat and jaw, supporting your sound with your diaphragm.

I hope that helped even the slightest bit.
Only play what you hear. If you don’t hear anything, don’t play anything.
-Chick Corea