well firstly don't expect the spells in Harry Potter to work. you cannot point a wand, say some words, and shoot lasers from it. real spiritual magick is different from fantasy hollywood magic.
spiritual magick utilizes energy found in nature to create change in your life. if what you want goes against nature [fly, grow wings/tail/cat ears, transform, or become anything other than human to name a few] will not work. a spell that deals in energy, or forces [luck, love, wealth, protection, curses, cleansing] do work because it's naturally possible to effect change in forces and energies.
you should start with meditation and energy work. focus and visualization are another important thing to learn. when you do finally cast a spell, be sure you can feel the energy flowing through you and pooling in the objects. visualize and focus on the desired outcome, then work to achieve your goal. a job spell will fail if all you will do is sit in your house.
look through the articles and try to find some books about magick to help you in your casting. practice before you cast, know what magick can and can't do, and above all cast when it's needed, not because you're bored, or want to show off, or any other arbitrary reason.