Finding a String

So, a somewhat ambiguous question here. I'm attempting to make a program that will insert code at a certain point in a file, the problem being that I have no idea how I would find the string.
I'd like to set it out somewhat like this.

--InsertHere	 <-Where I want to insert contents of another file.
<File Contents>
</File Contents>

I know how to pull the contents from another file and from the file i'm writing to, I just don't know how to find a specific string and write to that point.

Take a look at the documentation for the string.find() and string.gsub() functions here.