Try Doing A Slight Line, Then For The Knee, Twirl The Pencil To Make It Curve Out And Look More Real... Then Make The Calves A Little Skinny-Er Then The Thighs Down To The Knees, Then If They're Bare Foot, Just Draw Ze Foot, And Woolah! XD
It all depends on how you draw, personally i never got into drawing simple shapes and starting from there. In fact I am not very good at drawing humans from the top of my head...
...though I will admit that I have gotten better after drawing other people. As Sonny said, sketch other people's legs.
Learning about anatomy is a thing you should do in your spare time. I took anatomy class thinking it would help me become familure with the human body (which in turn would help me draw it better) but to be honest, it was a bloody mistake >.< If you want to learn about human anatomy go to a local library or something and do not take any classes unless your good at that type of stuff or your going into the medical feild.
Actually, NoName, that's a good point. When I said "learn anatomy" I didn't mean "learn medical anatomy", because this is very difficult. The only reason I took such a formal approach (and consequentially the only reason I have a nearly complete understanding of musculo-skeletal human anatomy) is because of the years I spend studying it as part of my professional studies.
If you're not interested so much in actively learning "what type of joint does what bone have with each other" "what kind of muscle attaches and where does it attach" and all that, then you're better off looking up nude references yourself for real-life examples, or else buying one of those "How To Draw Anatomy" books that take an artistic approach. Learning by example is probably more effective, anyway.
try drawing three vertical lines about half an inch away from each other but make the middel one shorter and for the knees wright u's in the middle of the outside lines and the inside line and make the u's wide