The key to drawing realistic portraits is shading. It can be hard to do realistic portraits using only lines. Try to focus on lighting and shading. =)
Here’s a random example from the net:
https://encrypted-tbn3.gstatic.com/images?q=tbn:ANd9GcSXwO17rOw4fd3S3AlThUuWlmcDuW9sPtwEXddgD_v6XrdQxz3Kzw
Notice how the shading creates dimension. =)
And to add, you should check out some anatomy books that can help you with form. You dont want to have a lop-sided figure, unless its what you style calls for. and even with that, you’ll need to learn the fundamentals of what you need to do
I tried experimenting with shading. It was my first time, so, I don’t know if it’s good or not. Also, I don’t know what, but I think there is something missing in this picture. It would be very appreciated if someone has a trick for hair.
I draw the portrait from this picture: img.mi9.com/female-celebrities/5084/emma-watson-2012-beautiful-girl_1920x1200_96539.jpg
Taito
6
Definitely study light and shadow, especially if you’re drawing from real life. Take a color photo you like, then desaturate in Photoshop, then visualize the photo in terms of black, white, dark grey and light grey (ignore the other shades for now). Find the old graphic novel Sin City for examples of drawing without using contours (lines).
rook
7
re: drawing hair – think of hair in blocks of volume.
Don’t stress with making out the individual strands when you blocking in the features… that’s to be done at a later stage.
Some of your linework is quite nice.
Since you are learning by drawing from reference, the best tip is to look closely at what you are drawing, and draw what you see – not what you think you see
Especially the eyes.
Next tip is to look at the pic in the mirror. Mistakes become evident that way 
And practice, practice, practice 
keep it up!
For portrait drawing a great book to check out is ‘The Artists Complete Guide to Drawing the Head’ by William Maughan. It’s a fantastic book and teaches you so much about using light and dark to create realistic portraits, would be a great investment 
you’re getting better! keep it up