There are several online resources that can help you get better with design software. One website I use as a reference is http://vector.tutsplus.com/.
Vectortuts+ is a blog of tutorials, articles, freebies and more on all things vector!
We publish tutorials on techniques and effects to make awesome vector graphics in programs like Adobe Illustrator and Inkscape. Our authors and writers work all over the industry.
They have posted a really good tutorial on how to draw a portrait as a vector image.
Quick Tip: Rapid Vector Portrait Process