Portraits can be more difficult than first thought to get right. As well as the positioning of you the photographer and the model, the focal length of the lens can play a big part in how the image looks.
If you don't have much room in your studio set up it might be necessary to use a wider lens, with a focal length of around 25mm. This will allow you to get more of the scene into the image if you don't have much room to back up, but it also means that there will be some distortion in your image.
For example, the model's face might end up looking larger than it actually is - this is never a flattering look! If you really don't have room to use a longer lens and move back, then a partial solution to the issue is to shoot from a slightly different angle. If your model is standing, then instead of focusing on the head, bend down slightly and focus on the midriff. this will elongate the head, helping the image to appear more natural.
A 50mm focal length is used commonly for portraits as it's the closest to what the human eye sees. It will reduce the amount of distortion in your image but distortion might still be present if you are close to your subject or a part of the body, such as an arm reaching out, is in front of the torso.
If you want the subject to be able to move around, or have parts of the body nearer to the camera, then you'll need to use a longer lens, such as a 150mm. This will narrow the field of view down, cutting out the unnecessary background in your shot. It will also result in a shallower depth of field at the same focal length, ideal for shooting on location.
Using a longer focal length, of 300mm or more, is ideal for getting a shot including only the head and shoulders, without any distortion. It will also mean that the background will be out of focus, even if it's just behind the model.
Experiment with different focal lengths and their effects to see what you can achieve - you can create some fun effects with wider lenses, too, such as shooting from above the subject to give the effect of zooming upwards, with tiny feet, and getting your model to hold their hand out, so it appears massively larger than the body.