Textstream Objects

What I'm trying to do is take a really long text file and just pull out key lines in the file based on the first word on the line to create a new text file with just the lines I select for written.

I'm comfortable with the opening and creating textfile part, but how would I scan through the source file and only readline when I hit line that starts with a keyword, then append that to the new file and keep going until the next keyword or file end? That's what I can't figure out.