When you are on a box it should feel the same as just standing on your board in your living room. Completely flat.
For the jumps you want to watch other riders and the motions they make. If you see someone that is doing good strait airs ask if you can check their speed by following them on the approach. Don't follow them over jump but just go to the side.
What angles you ride is completely up to you but I do notice alot of new people ride a more narrow stance then is ideal. I'm 5'8" and started out at under 20". Moved to 21" after a few weeks last year and got a big improvement in my riding. Went to 22" for this year and it has been even better. With being 6'3" I would be suspect of a stance under 23". That's measuring the distance between the center of your binding mounting disks.
Videos to help you.
Park rules Smart Style in the terrain park | SnowProfessor.com
Jumping YouTube - Introduction to Jumps
First snowboard jumps | SnowProfessor.com
Boxes YouTube - Introduction to Boxes
How to 50-50 a funbox on a snowboard | SnowProfessor.com