it's hard to say really. you need to believe. i have a friend who doesn't cast because she doesn't believe in it. myself, i'm rather logical, i've seen results for every spell i've ever cast in one form or another, but i have my suspicions, so i typically stick with potions and brews because they rely on herbs and i know the magical and medicinal qualities.
before you can cast, you must learn to charge and ground. visualize energy entering your body, most people do this while meditating, but you can dance, sing, drum, whatever, as long as you can feel the energy then it's working. once you're done your spell, you must ground the extra energy or else it builds up and you won't feel well because of it. typically, i tap the ground with my hands [front and back] but i have a friend who stomps his foot [first time he did that scared us all because we were meditating lol] point is, you need to visualize/feel the energy left over leaving your body and returning back to the earth.
once you've gotten the hang of that, you can cast. be sure to cast spells you need, not spells you feel like casting for fun, you're bored, or want to test your skills. to start, let's say you cast a spell to help your grades on an upcoming test. be sure to study, because spells WILL NOT work unless you do something productive to help them along. keep in mind, you won't get an A+ or anything, but you may see like a 10-20% increase on the test.