Mate put your iso at 100 and use long exposure.
Read what link provided by @Francis247, it is the best smallest and to the point article you can read around.
And play around with your camera, you can learn faster from hit and trail. There is no one rule which fit in all situation.
Don't bother using AUTO ISO, at least if you want grain free pictures.
u are using manual exposure? maybe that's why.. use shutter priority or aperture priority mode instead
for proper exposure, u need to use the correct metering.. u have spot metering, centre weighted metering.. for my 500d, the evaluative metering is not accurate sometimes.. then u can use spot of centre weighted metering.. aim at the grey (mid tone) areas, lock the exposure and then shoot