Your question is a little unclear. Does your 'lines' layer already exist?
If you want your layer to be a featurelayer rather than a graphics layer, then create a featureclass in a workspace, and add your line object to it like this (sorry it's in vb.net):
Dim pFeature as IFeature
pFeature = pFeatureClass.CreateFeature()
pFeature.Shape = newPolyline
pFeature.Store()
Then create a featurelayer from your featureclass and add it to the map:
Dim pFeatureLayer as IFeatureLayer
pFeatureLayer.FeatureClass = pFeatureClass
pFeatureLayer.Name = "Lines"
Dim pMap as IMap = pMxDoc.FocusMap
pMap.AddLayer(pFeatureLayer)