From what I've learned here and through reading, set it up and start twisting the string till it both sounds acceptable and the groupings are acceptable. Do some shooting and then play around with the brace height some more till you get it just where "you" want it. Different styles of shooting, releasing, etc all have an effect on how your bow behaves.
As far as for arrows, what kind do you want to shoot? Woods? Aluminums? Carbons? Decide what kind you're going to shoot then we can talk about spine and lenght.
Enjoy the journey!