Help rewording this sentence!

The only change that you need to make is to get rid of the comma. Otherwise, it's a grammatical WH-cleft sentence and sounds natural for spoken English or informal writing. If there's no special reason to emphasize the second part ("something no one could..."), you could reword it more concisely by getting rid of the cleft altogether: "I was about to get myself into something (that) no one could have prepared me for". The "that" is optional, and "something" could be replaced by another noun, such as "a situation".