What is the method to seal a concrete roof?
Concrete itself is not waterproof, in fact, it's more like a sponge, so concrete alone is never used to create an impermeable surface. You haven't provided much info - is the roof flat, sloped? What is already up there?  There are tons of different waterproofing methods available.

Going under the assumption that it was properly waterproofed at some point, you need to figure out why it's leaking.   Maybe there is damage to the roof, or maybe there is a drainage problem and water is just building up and finding its way in. Also possible is that the flashing around a vent or access hole has been bent away and water is getting behind it. Either way, you will first need to investigate and resolve the root problem and then fix it in-line with how it was already constructed.  If the roof is tarred, you are not going to put down some other barrier.

You will without a doubt have to go on the roof to check it out. If you're not comfortable with this, its best to call a roofing expert.