Wanna go Platinum

The easiest way is to bleach (with hair bleach) until your hair is the color of the inside of a banana peel.  After you wash that out, use violet toner over it to remove the yellow and get it to white.  You can also buy violet shampoo (looks like purple paint in the bottle, not the translucent ones).  Sometimes toners fade and your hair can wind up yellowish again - using the shampoo will keep it white.  Do test strands...with hair this short, clipping off some and doing that on a plate is probably the easiest way (just mix up a little of what you'll use, don't mix it all in advance for the test strand or it won't work later).  Test strands will help you get the timing right for the bleach so that you don't overdo it and burn your hair, and also testing the toner will help you make sure you don't turn your hair too grayish.