How to select 'Blank colour in Photoshop?

ok
first go to windows & make sure your layer options is active
their are 2 option in layer window called Opacity & fill as you will reduce their value from 100 the colour will start becoming translucent & finally at 0 it will become transparent

& pls write in details what do you want to make..............

see the sample attachment

use the eraser
it makes blank, right

When starting a new image itself, we can assign a background color of transparent, too, if that's another doubt.

ok , thanx Superman . it worked

rep me

try this too:
Go to File>New> and then, in the background contents, select transparent.