The Trademark Office publishes a manual called the Trademark Manual of Examining Procedure ("TMEP"). The TMEP is freely available online and contains most of the information that trademark registration applicants need to know about how to go about persuading the Trademark Office to register a trademark. There is an enormous amount of information in the TMEP that can only really be appreciated by -- and is really only useful to -- attorneys with knowledge about trademark law.
Nonetheless, you can view the TMEP at Likelihood of confusion rejections are discussed in Chapter 1200 at Section 1207 and Use in Commerce has its own chapter at Chapter 900.
If your registration application is important for your business, then it makes NO sense to respond to the Trademark Examiner yourself. Depending on the facts, it would take a trademark lawyer about half a day or so to draft a reply to the Office Action rejecting your application (assuming there are any arguments to make of course). In short, you should read up on why your application was rejected and then contact a trademark attorney to respond to the Office Action if the registration is important.
I absolutely agree with Mr. Ballard’s advice about seeking the help of an experienced trademark attorney. A lot of my clients tried to save money by doing it themselves only to find that the easy part was filing the application. It is inevitable that you will receive an Office Action from the Examining Attorney and to a lay person they might as well be written in a foreign language.
If this trademark is important to you, then you need a trademark attorney.