Should i plea guilt or non guilty?

i did something stupid and got caught stealing a 289.99 zune. i didnt get arrested due to full cooperation with the store, the cop just wrote me a citation and a date to apear in municipal court, whats the best option to plead guilty? or not guilty? and just for my knowledge do thy inform u on ur punishment before pleading or not? im 18 by the way and have had a clean record up till now, its my first offense of any sort.