It is better to ask a Congressional caseworker to make inquiries for specific information or to redress a particular violation of regulations than to make general requests for help. Congressional military caseworkers have varying degrees of GI rights advocacy experience and have other responsibilities. It is important to stay in touch with them. An Article 138 very commonly begins with mistreatment by a chief, a division officer, etc., under the commanding officer's authority, and the initial request for redress may be against a member of the command, not just against the commanding officer. The complaint portion may therefore be for the commanding officer's failure to control a subordinate.
Relief can include a request for an apology, a training or briefing about policy, etc.
The Army and Navy provide sample formats for Article 138 complaints in their regulations, and sample letters of complaint can also be found at Sample Article 138 Complaint. Be sure to date all correspondence.