Usually white people are a good place to start, theyre nice easy targets and are the most generally accepted to race on; australians, irish, russians, italians and french will probably give you something to work on for the first couple of weeks while you get started.
Once youre comfortable being a little racist here and there you can move onto bigger game like arabs, chinese, mexicans, indians and people from south america. This part can be tough since you just got on your racist feet and youre not used to interacting with those who dont take kindly to racism. Dont get discouraged!
Finally youre a real weathered racist! youve got the tattoos to prove it and a few scars from victoriously racist battles in the thick of it. Time to move on to the holy grail: the blacks. These ones will really test your racist abilities, but as long as youre white, you should have at least a couple hundred years of leverage against them. If necessary, group up with other racists with different skillsets, such as backhanded comments, muttering under breath, or ROTs (racism-over-time abilities).
Its an uphill struggle to be a racist, but i really think with the proper time and dedication, in conjunction with a proper diet and exercise, racism can be within anyone's grasp.