Well, you actually breathe from your diaphragm naturally. Lay down on your back and breathe normal, you'll feel your stomach rise. That is breathing from your diaphragm. In chorus we do breathing exercises where you make a triangle with your hands on your belly (thumb tips touching each other and pointer finger tips touching to make the point) and breathe in deep and let it out. It helps you to focus on breathing more deeply with your diaphragm. Do that three times then breathe in and sigh three times. We also do sirens.. Low to high, high to low, then low to high back to low… For high pitched notes, it's all in breathe support… You just need a good amount of air… To make yourself louder, exert your air more by using your abs like you're sucking in your stomach lol. Also, you might not get high notes because you could be an alto (altos sing lower notes… But are able to sing higher than tenners… Or something like that) If you have any more questions about this you could do like an add on or email me or somethin' >w< Hope I helped!

You can read the tips to become a better singer,here:

Http://www.becomesingers.com/singing-tip…

There is not way you can become a great singer instantly. It takes practice. To help your singing i suggest doing vocal warm ups every day. And after a week or two sing a song for your family and ask how you sound.
Acting… Once again. It takes practice. But to some it just comes naturally.