I'm going to be point out some mistakes, so if I sound a little too rude, I'm really sorry, I tend to get out of hand...
For the first picture, the ears are too small, and are placed below the eye, which is wrong. Take your thumb and your index finger (the second one) and place your thumb on the base of your ear, while you place you index on the top. Then run it across your face until it meets an eye.
You will notice that your index finger reaches right above your eye, and below the eyebrow, while the thumb is at the level where it is just above the upper lip. You can use those as measurements in the future.
The left eye (what we see on the right) is also a lot thicker than the right counterpart. This can be a common mistake, so don't fret about it and continue practicing.
As for the full body, try not to place the arms too wide apart. (See: Shorter Shoulder width)
The arms can also do with more length. The arm consists of 3 parts, the upper arm, fore arm and wrist. The upper arm reaches to slightly above the hip, but only slightly. The forearm goes down to half the thigh. The wrist goes down the slightly below the the third quarter of the thigh.
As for the last picture, do you think that the head is a little too big? Other than the points I said above, the neck seems to be a little thin, and the arms a little too stubby. However, if you were aiming for your character to be fat, the stubbyness is ok to stay.
This site might be a little difficult to follow at times, but you can always try. It covers the entire thing is greater depth.
PolycarbonGood luck with your future endeavours.
Don't worry, I was much worse 3 years ago. Drawing well takes years of practice. (Unless you use up a pen in say, a week. Not for writing, but for drawing.)