A list of U.S. state abbreviations can be found on a number of websites such as StateAbbreviations.us or on one of the 50 official state websites. State abbreviations are especially necessary for mailing addresses.
The most common type of U.S. state abbreviations is the two letter postal abbreviation. States are usually abbreviated by taking the first and last letters (or second letter) of their names or taking the first letters if it has two names. For example Colorado is abbreviated as CO, Alabama is AL and New York is NY.
If a state name is not abbreviated as the first two letters then it is usually abbreviated by taking the first letter and a letter in the middle of its name. For example, Tennessee is abbreviated TN.