Those are pretty easy questions to answer in an e-mail, but for knitters who want to design their own sweaters, it gets a little more complicated. A great guide to the basics of designing your own knits is Design Your Own Knits in 5 Easy Steps by Debbie Abrahams.
This book provides a good overview of subjects such as getting inspiration, preparing a rough sketch of your idea, swatching, refining your pattern and knitting the final garment. It offers general schematics and design ideas for a variety of basic shapes including pullovers and cardigans with set-in, raglan, drop-shoulder or saddle sleeves and a variety of necklines and collars.
It's not as detailed as some design books out there, but that's probably a good thing for those new to designing because it doesn't make designing your own projects seem at all intimidating (though there's clearly math involved, it shouldn't scare you).