diffculties using Manual mode with Canon DSLR 650D
There is no global setting that works in all these conditions. The solution is to light properly and expose properly for the conditions. It's hard to give specific advice because you don't say what you're shooting in these low light conditions or what venue you're shooting in. Here are the general solutions:


Do you have the option to increase the lighting? If so, that's you're best bet. Note that you may not need to buy more lights - reflectors can do wonders and are very cheap.
If you can't increase the lighting, the next best thing is to open the aperture. However, that causes your depth of field to become more shallow, meaning it's harder to focus on your subject and keep them in focus, particularly if they're moving.
Decrease the shutter speed. In video, it may be called the "shutter angle" and it may be in degrees. 360° is the entire length of a single frame (1/24th, 1/30th, etc.). 180° would be half that (1/48th at 24fps, 1/60th at 30fps, etc.). You can only increase it so much, though, as a frame can't be longer than 1/30th of a second (if you're shooting 30 fps, for example), so that's the upper limit.
Increase the ISO. As you've noticed, this causes more grain in the image.
Use a camera with a better sensor for low light conditions. Since you're into Canon gear, the 5D is known for its large sensor size. In general, larger sensors (that is sensors with larger pixels) will gather more light and produce less grain. There may be cheaper options. You can rent better gear much more cheaply than buying it in most parts of the world.
Use noise reduction software in post. There are lots of plug-ins available for the major editing applications. I've seen people use Neat Video, De:Noise, and other similar products.


If you describe what you're shooting and where, we may be able to come up with more specific suggestions.