The 35mm lens is not the best choice for portraiture, as the lens does have some perspective distortion. Perspective distortion is what makes faces look round and noses look big.
And why are you are using the 18-55mm lens in manual? Manual mode in lenses is typically only required for specific situations. If it is Nikon lens - the one that probably came with the camera - it should say AF-S in the model name, and it should autofocus on your camera. AF-S lenses have internal autofocus motors, so it does not need a camera focusing motor.
If it is AF-S, and it isn't auto focusing, it is probably broken. Hard to tell with used equipment.
If that is the case, your first step is perhaps replacing the 18-55mm lens. While some photographers do use a 35mm prime as their only lens, most use the 18-55mm as it is more convenient. But used correctly, either lens can produce good results. You can get a replacement 18-55mm for around $125 or so.
It is also possible - with used equipment - the previous owner set the camera up for spot focus and set the focusing spot for somewhere other than center. So the result is the camera may be trying to focus on something other than you. To ensure the camera is not setup weird, look in your manual for the master reset and use it. This will set the camera back to factory settings so you do not have to look through each setting and wonder if it is set correctly.
DSLRs are very powerful cameras and have many settings to allow a skilled photographer to setup the camera for the best photos. But this presents a problem to the novice as they can inadvertently make a change that will do something unexpectedly. This will self-cure itself over time as you gain experience with the camera, but for now, using the factory settings would be your best approach.
This is one reason I never recommend used equipment or even buying on-line for novices as they can become overwhelmed. And if you buy new and have a problem, if you bought from a camera store, they can help you. Especially with camera equipment, the camera companies usually have strict pricing rules for dealers, and you cannot buy a camera any cheaper on-line than at a camera store (at least in the US).
And the exposure issues you are having is because you are not setting your exposure correctly - or perhaps your camera is broken too. It is not a lens issue.