I like to use commercial stiffener for my crochet pieces. I use Stiffy (hee hee)--it comes in a white bottle with a pink label and you can dilute it to control how stiff it will make your work. You can use glue thinned with water, but as time goes on it will usually turn yellow. Sugar syrup will work but it gets sticky in high humidity and will wilt if it gets wet. I've used a corn starch solution for doilies but it will also wilt if it gets wet.
Kids Crochet: Projects for Kids of All Ages has excellent instructions on single crochet, increasing, decreasing, etc. It doesn't teach you how to do any double crochet, etc, but if you're mainly interested in amigurumi sc is all you need. The patterns are sized for kids, but the pillows, bags, and toys are cute.