Get advice and free legal help on custody and child support issues (and more) through Maryland Legal Aid. Set up an appointment at one of their 13 statewide offices by calling one of the phone numbers listed here. You can also apply for legal help on line.
You may be able to get free legal services from one of the lawyers who volunteer through the Pro Bono Legal Assistance program run by the Bar Association of Montgomery County. Only low-income residents of Montgomery County qualify for these services, but the representatives at the offices will help you find someone in your area. Call them to request an application here.
Another group of volunteer lawyers works through the Maryland Volunteer Lawyers Service. To find out if you qualify for free legal representation, go through the application process here.