There is a certificate which is to prove you're eligible to National Health Insurance Program which is one of SSP(Social Security Program:사대보험) for Korean resident. You expats are sometimes refused because of your ID is not registered at the system, so you need to bring it and submit it to the reception whenever you check in, just take a picture by your mobile and let your reception girl see it.
Some time they may ask you to see the ARC(Alien Resident Card: 외국인등록증) whether it's match with your ID or not to avoid any misuse it by others.
You can join this program when you're hired by an employer(we call it "Work place program: 사업장 가입자") with your dependents without sure charges, or you can also join by yourself(we call it "Area program:지역가입자").
If you're a sole proprietor without any employee, you need to join the Area plan since you're not eligible for the work place plan.
For area plan, you need to stay here more than 3 months consecutively before your application if it's the first time ever in Korea.
Jz also provide services for Area plan as well as the Work place plan.