In Spanish, CH is considered a single letter.* It’s pronounced like the ch in the English words "chat" and "chess."
* It's very important to be aware of this when looking something up in a Spanish dictionary. The CH section is between the C and D sections. For example, chico is located after como, ocho follows octubre, etc.
Please note that this explanation is only a guideline for the Spanish that I know, which is Castilian Spanish. There are many regional variations in Spanish pronunciation.