もちをください。 "Mochi o kudasai" would be a polite way to say something like "Please give me mochi." You could also say もちおねがいします。 "Mochi onegaishimasu" which would be like "Mochi, please."
Here are two pages I found that may help you:
http://www.zonjineko.com/3570-5-steps-to-ordering-a-meal-in-japan/ and
http://www.guidetojapanese.org/blog/2010/01/30/ordering-food-in-real-japanese-part-1/
Hope that helps! : )
February 2, 2013 at 12:12am