When you set the camera to manual you have to set aperture, iso and shutter speed.
Until you know how to make those settings you should not set the camera to manual.
To make those adjustments you need a light meter. The camera has a light meter, so use it.
Here is a review of the lens. I suggest you read it:
Im a complete newbie and want to learn how to take pictures in manual mode. I have a manual lens (zeiss loxia 35mm) on a Sony A7 camera. When i put the camera setting to manual, the pictures come out dark. What im asking is, if i leave the camera setting to auto but use the lens which is manual... will i have full manual control of my pictures? Or is there a better option?