I actually was in the same situation as you a little while ago. Lately I've been working on my 3pt shooting. My best advice would be to make sure you're using your legs. Your legs should provide most of the power behind your shot. Really bend down and flex at the knee. However, your dad is right. You should finish with your shooting arm high and straight and with a hard wrist flick. This will put backspin on the ball which helps when it hits the rim. Just keep practicing these tings over and over again and you'll get better. Also, you could search around this site for good articles on how to improve your shooting form. I hope this helps and good luck with it.

make sure ur using your finger tips.

this will help your game alot! trust me, im a tall skinny big man and most opponents dont expect me to be a threat from outside so the back right off me and i score about 6-8 points per game on outside shots alone simply because defenders werent expecting it. get your long range shot stroaking and youll notice a big difference. good luck