JOSM is an advanced OpenStreetMap editor that is popular among experienced editors thanks to its plugins and stability. You can read more about JOSM at the OpenStreetMap JOSM wiki page and from the JOSM section on learnOSM
A roundabout is a road junction where the traffic goes around in a one-way circle. Often roundabouts are circular, so you can use the circular tool in JOSM to draw the basic shape. Also, add the junction=roundabout tag to the roundabout. Then you can easily connect the ramps to the roundabout.