How do you break codes?

One way is to use a letter frequency table, to determine which letter each symbol represents. In English, for example, the letter "e" is the most common, so if the code your looking at was composed of numbers you would find the most common number and assume that is an "e," then move on to the next most common using your letter frequency table. If the message doesn't start to make sense you try a few of the most frequent letters in place of the most common letters, numbers, or symbols used in the code. Step by step you break it down in this way.