Roundabouts are tricky at first. It took me a while to build confidence on them too. The trick is to just look left to see if there is either any cars moving out of the right lane or coming around from by the right lane. A good measure I find is if the car coming around the roundabout isn't at the lane on your right then you should move out. Of course, keep an eye on their indicators too.
I found driving to be largely about confidence. As you gain confidence you become a more assertive driver.