Another option is using a visual oriented version like 4nec2 by Arie Voors. http://home.ict.nl/~arivoors/Home.htm
His version has a couple of quirks but being able to draw your antenna while seeing it in a 3-D modeling environment goes a long ways towards understanding what is going on. With his version you can draw your antenna and then switch to a NEC input view and see the cards that were generated. I have learned a lot (but I am no expert) from using Arie's version. Warning, it is addicting.
Also, be advised that the free versions are based on the NEC 2 engine which isn't the most accurate engine and has a number of limitations. The NEC 4 engine is much better but you must purchase a license to use it for about $350, if I remember correctly.