in some practices, students are to learn for a year and a day before they are allowed to cast spells. Most people are too impatient, or simply dont know better, and jump right into spell casting.
Odds are your body simply doesnt have the energy for what your trying to perform. This site has many stickied threads on the basics, study them and learn to get in tune with your energy.
As for wanting to be able to hurt people a Little.. most people will tell you magic is the same principle as martial arts, learn to better yourself not to harm others. Take that or leave it, the bottom line is this: you Have to mean it.
You can't walk into magic and only half care, or do a spell and slightly want something to happen. If you dont want it, dont do it. Because if you can't Mean it, then you have no buisness casting the spell.