Best thing to do is to go to a newsstand or small shop on most streets (will sell cigarettes, soft drinks and all kinds of little things) and say sim - ka.

If not (ie you dont' feel like it, or are intimidated), check into the hotel and ask the front desk and concierge to help you and if there is a place nearby where you can buy one. Say you want a local sim card and could they help you. Just to make sure, say you want a card like theirs. Should be simple.

The instructions (at least for China Mobile) if I remember correctly should be in Chinese, so if you're stumbling with that, once again the front desk and concierge should be happy to help. Very simple stuff. No need to worry about it.