You can either trade, or breed.
save before you choose your starter and pick the one you want. If it is not shiny then soft reset - L+R+SELECT+START all at the same time - this will make it go back to where you last saved. Keep going till you get a shiny starter although this will take a while as it is a 1 in 8192 chance of getting one and even then you are not certain to get a shiny.
Hope this helps :)