It will help if you address your letters and parcels in Chinese. The post office has staff who will translate addresses, but this will add to the elapsed time taken for your letter to reach its destination and may not always be accurate. The postal code is required to mail a letter within China.
Stamps are not self-adhesive and you will need to glue them to your letter or parcel.
The postal staff may want to inspect the contents of any parcels before they are sent, so it is advisable to seal packages while you are at the post office. Normally, CDs and DVDs will not be accepted for posting.
Many international courier companies operate in China, including DHL, UPS, TNT and FedEx.