Who is telling you this error?

I checked my site using my google webmaster tools account and this is how my robots.txt is built. It works fine.

User-agent: *
Allow: /

This info seemed informative. The Web Robots Pages

I used this checker New Robots.txt Syntax Checker: a validator for robots.txt files !

So how do I input the disallow command?

eglassman88 is right, crawling happens by default.

This is a robots.txt entry that blocks the whole site: (Disallow everything)
User-agent: *
Disallow: /


This is a robots.txt entry that allows full access to all robots: (Disallow nothing)
User-agent: *
Disallow:

The complete blank after the "Disallow:" indicates it's disallowing nothing.
You should really take advantage of Google webmaster tools, they can really help you with crawling errors, what should be blocked, and obviously much more then just the site access issues.

FYI:
Google and Bing accept 2 specific regular expression characters for pattern exclusion (* and $).

Here's the minimum I start with in my WP sites:

User-agent: *
Disallow: /wp-*
Disallow: *.php
Disallow: */feed*
Disallow: */trackback*
Disallow: /cgi-bin/