Start at ISO100 (forget about 50). Put the camera into aperture priority mode. Select the aperture that gives you the depth of field (or lack thereof) that you are interested in for the image. Activate the metering and look at the shutter speed, if it's not at least 1/2 x focal length (i.e. 1/100s for a 50mm lens) then increase the ISO until it is, or use a tripod.