1. Remember, people have skulls! The skull is the basic form you need to work from. Once you know the skull, you can then place the features where the skull wants them to be! The eyes go into the eye sockets, the nasal structure is based off the bridge of the nose, ect.
Currently, you draw the eyes too far apart. There should only be an eye width between them! Rule of thumb, the eyes should be pretty much the middle on most people. Toddlers and infants have a different facial structure, but we won't go into that right now.
Next, the top of the ears are about the middle of the eyes. The bottom of the ears is usually the end of the nose!
For the jawline, make sure you can fit a nice healthy jawbone in there! Your character needs to be able to chew his food properly!
2 & 3. I'm going to merge these two because they're similar in nature. The limbs are a bit long. The hands should end up falling about the crotch region. This way people can easily undo their pants when they need to use the restroom! The legs should be as long or a tad bit longer than the torso. Humans are pretty evened out.
4. Make sure you know your muscles! The pectorals don't show through clothing unless it's a really tight shirt and the person is really fit. Check out The Rock in most of his movies to see what I mean. Or like Hugh Jackman. Either of the two works really.
The trapezius muscle connects to the shoulder like this(Ignore the drawing to the left. I mostly needed the right image.):
5. Remember, the human body has form! It's not a 2D shape! Perspective and form are key. The human body can be broken down into simple forms, such as the sphere. Really, if you can draw forms, you can easily draw the human body. Just remember perspective when you practice the forms.
I drew the pose on the right for more of a visual form thing. Don't take it as law! The only law is life drawing!
I'd recommend reading Andrew Loomis' book, "Fun with a Pencil."
You can get it for free here: http://www.alexhays.com/loomis/
I'm really looking forward to seeing how you improve, mate. Keep it up!