Blizzhackers

I have an application that I am trying to hack.
Before it sends data to the server, it encrypts it.
Using OllyDbg, I found the address of the encryption function.
When the encryption function is called, the unencrypted string is one of the parameters passed to the function.
I want to log this string.

I replace the call to the encrypt function with a call to my function, that writes the data to a file, then calls the encrypt function and returns like nothing happens.

One problem. Where do I write my function? I tried hex editing it into a code cave, but it was not very successful. Do I need to "hook"? Once I am in the address space of the application, what do I do? I know I can read and write the processes memory, but how do I "intercept function calls" and read the parameters passed?