A few things I've picked up from my singing teacher:
Expanding your pitch range comes with practice, you can't skip this.
Many people when beginning to learn have their mouths open wider than is natural. Mine myself was actually more closed than ideal. Watch someone with experience sing and try and find the correct balance.
In the pitch you are most comfortable with, sing as loud as you can while staying clear. This is the tone you should attempt to maintain higher and lower.
Hope I can help