Advice number one: start singing at home

I know it can be difficult, but you have something great going on so why be shy? You could always close the door and pretend you are home alone if that helps
I'm sure if your family knew your plans and heard you sing they'd be supportive and delighted
Advice number two: breathe and open up
It's important to breathe with your stomach. Make sure the air goes all the way down. When you breathe in, don't gasp (I hate that noise

) instead just drop your jaw and air will flow in naturally. To reach a higher range you will need to open your mouth widely, sounds a little silly explained like that but try it, it helps

... And don't hold back.
Advice number three: stand up straight
Think about your posture. You need support from the feet, from the belly and from the back. If you sit, sit properly.
I hope this helps