Robots.txt And Add-On Domain?

I've never concerned myself in the past with a Robots.txt file being included with any of my domains, but recently I've been reviewing a couple of SEO ebooks that seem to imply that they are necessary for proper and efficient optimization.

After studying and reading up on robots text files I've managed to confuse myself thoroughly.

So, here's what I want to do:

I have a root domain 123.com

Addon domain is abc.com (abc.123.com)

I need to have a robots.txt file that will only allow permission to index the addon domain (abc.com), and not the main domain (123.com) it is used for personal files and server scripts. I do have a WordPress install on the root 123.com though.

I've created a new site in a subfolder of the addon domain abc.com/new site.

I want to give permission to the search engine bots to search this 'abc' 'root' folder and any other additional sites that I create in this directory abc.com/additional sites site.

Where would I place the 'robots.txt' file..... Under the root 123.com or under the addon site abc.com?

Again. I don't want the subfolders of the root domain with the exception of the WordPress install to be searched or indexed. Everything in all the addon domains will be fair game for the search engines.

And, could somebody give me an example of what the contents of a properly constructed robots.txt should be, other than 'Allow'?

Thanks,
Mark