So this is usually something you just pick up with experience, but all the render states are located in the docs and you can see what they do. For example, the states you are using are:

D3DRS_CULLMODE, D3DCULL_NONE: This disables culling, which means any geometry drawn will actually get drawn, instead of being clipped or rejected. This works for you because the backfaces of your alpha quads are most likely being rejected.

D3DRS_ALPHABLENDENABLE, TRUE: This enables alpha blending, alpha blending is dictated by a function on how to blend the color's using their alpha. There are a lot of blend functions, so use this page to get the right sort of blending to work for you

(http://msdn.microsoft.com/en-us/library/windows/desktop/bb173417(v=vs.85).aspx)

Good Luck!