To answer your second question, reading and practicing the basics first is what you need to do. Learn about grounding/centering, meditating, visualizing, and energy manipulating. There articles on the site on the basics and there are also books on witchcraft that will include the basic steps to begin with. As for your first question, when you are casting not only are you using the energy of the universe but you're mostly using your personal energy so to cast several spells within the same hour/day will drain you so much of your energy. It's best for you to take your time with spells and concentrate on one at a time; some people can cast more than one spell in a day but since you are beginning it is best to concern yourself with one spell at a time as well as studying and practicing the basics. Sorry to leave a long comment, I do hope this helped though.
With experience of magical force, you'll get a better feel for when it's practical to cast a lot of little spells or a few big ones spaced out over a week or a month or something. Depends on the spell, depends on the caster.
I'm a great fan of this tutorial: http://thebookoft.wordpress.com/knowledge/witchcraft-from-the-ground-up/
Also, anything written by Christopher Penczak, "Instant Magick" and "The Witch's Shield" for starters... "Spirit Allies" and, if it's your interest, "Urban Magick".