arturas,
Welcome.
I’m glad to see you’ve decided to post a Sketchbook here. I know from your General Discussion post that you are a high school student seeking to explore 2D art. As I mentioned there, I think that the number one thing to focus on, if you can, is Life Drawing. Do you have access to Life Drawing classes in your area? If so, I would encourage you to attend.
Ultimately the best way to get better at drawing figures is doing Life Drawings and Paintings. Another great way to learn the figure is to do Master Copies.
There are a number of great Workshops here on the Forum which I would encourage you to check out. While these Workshops are officially concluded, you are still welcome to browse through them and to use the Reference posted at the beginning of each thread to do your own Master Copies.
A Master Copy is essentially your own drawing or painting, be it digital or traditional, of a work by a master artist - for example, Michelangelo. A Master Copy is not a tracing of the original work, but rather your best shot at duplicating the work of the master work - the benefit is that you really learn to see through the master’s eyes, and after doing many, many master copies, your own work will improve. It is recommended to do a mixture of Master Copies and Life Studies, and eventually, drawings from your imagination, over the course of your training as an artist.
Here are the links to the Master Copy Workshops which I think you will enjoy checking out:
Anatomy Lesson Series: Body Part 1 - The Head
http://forums.cgsociety.org/showthread.php?t=375031
Anatomy Lesson Series: Body Part 2 - The Torso
http://forums.cgsociety.org/showthread.php?t=405198
EDIT: You are also more than welcome to join in on the current Workshop:
Open Figure Drawing Workshop - Mixed Masters - 020
http://forums.cgsociety.org/showthread.php?t=437456
It is also very useful to pick up one or more Anatomy Books. Here is a list of books which I recommend for anyone interested in studying the human figure:
Anatomy Resources : BOOKS
http://forums.cgsociety.org/showthread.php?t=257424
I look forward to seeing more of your work, and seeing your development here. 
Cheers,
-Rebeccak