You go early on a Monday morning. In the United States some clinics have enroillment periods at the beginning of the fiscal year. Others have open enrollment.
They will schedule a meeting with a social worker who will determine your rate per visit and whether or not there is a co-pay.
You should specify where you live so those in your area can respond. In Quebec, Canada we have Local Community Service Centres (CLSCs) that are free of charge and have nurses available.