30kph or so. Depends on the roundabout and whether you can see clearly though.
As Snowfox said, don't forget to indicate off of them (even the small ones). Also if you mount the roundabout (even with one wheel) it is an instant fail during your driving test.
Just go as slow as you need to, other cars can wait. If I am turning left/straight through I steer with my right hand and use my left hand on the indicator. When When turning right I steer with my left hand and use my right hand on the indicator.