Generally speaking, most (nearly all) medications that are available in Western countries can be found in China. Nearly all of the common Western OTC (over-the-counter) drugs can be easily purchased at a Chinese pharmacy. Their distinctive green cross sign easily finds pharmacies in China. This will be prominently displayed on the outside of the building and will be an indicator that a licensed pharmacist is on duty inside.
English language skills can vary quite a bit; in large cities it will be possible to find an English-speaking pharmacist or pharmacy tech, but in smaller cities this might be more of a challenge. To get around this, try writing down the technical name for the drug instead of the brand; for example, Ibuprofen instead of Advil, Acetaminophen instead of Tylenol. Most drugs will require a prescription from a Chinese licensed doctor, although there are some antibiotics available OTC that require a prescription in Western countries. In general, so long as you head to a licensed pharmacy with a prescription in hand or with a little preparation; pharmacies in China are very similar to those found in your home country.