You can use the .bindPopup method to bind a desired attribute to the popup, with the use of an onEachFeature function.  See below.  Examples can be found [here][1].

    function onEachFeature(feature, layer) {
        if (feature.properties && feature.properties.NAME) {
            layer.bindPopup(feature.properties.NAME);
        }
    }
    
    geojsonFeatures = "county_coords.geojson"
    
    L.geoJSON(geojsonFeatures, {
        onEachFeature: onEachFeature
    }).addTo(map);


  [1]: http://leafletjs.com/examples/geojson/