how can i make a promo code picker for my app

hi guys trying to make a promo code picker for my app. i have a list of 10 promo codes in my app. few of which i have displayed below.

H20_FOR_THE_WIN, H2o_ADMIN_RULE, I_LOVE_H20 

and so on. when i press a button i would like it to random pick of 1 of the 10 promo codes. at the moment i have just 1 promo code. but i think this could be easily cheated can some body point me in the right direction to implement something similar.

thanks guys you are all brilliant