Tommy, if you've made the leap from cartoons to realistic people, then you've done very well with this piece.
As airscapes has said, to achieve realism and likeness in portraits you have to be careful and measure, check your proportions and recheck again to ensure main features line up and distances are correct.
Looking quickly at this and not seeing a reference (I have no idea who Kelly Brooks is by the way

) I would say that the eyes may be too far apart and you need to look at the angle of the viewer's right eye as I don't think its at that steep angle. Also I think the distance between the nose and mouth is too small, which alters her facial expression.
The other thing that jumps out at me in the drawing is the lack of contrast in values. You have some shading, but it looks like blending and most of the shading is all the same value. You need to get some stronger darks to make the portrait stand out. Convert your reference photo to greyscale and compare your drawing to it. You can then see how dark you can go with your values in your drawing.
I advise against blending for those just beginning portraits as its easy to become too reliant on it and use it constantly. You never get to fully understand the potential of shading with your pencil alone if you always blend to a smooth tone. You also fill the surface of the paper more quickly if you blend a lot too.
Common mistakes in fan art such as this is that people concentrate on features that appeal to them and gloss over the areas they are not so interested in or lose interest once they've drawn what attracted them in the first place.