first meditate [outside if possible] once you learn to clear your mind. when ready, visualize white light entering your body. i can be when you inhale, or washing over you, or whatever, just visualize it. visualize it enter your body, taking place of dark energy/light. feel your body fill with the white light until the dark is all gone. [inhale white light, exhale dark] remember, you should be able to feel the energy, you might feel more awake, or hyper, it's different for each person. it's hard at first, but it'll get easier over time. as you go throughout your day, be alert, see if you feel differently.
when you're ready to cast a spell, try a simple candle spell, take a candle of a coresponding colour to the spell you wish to cast [blue=healing white=peace] sit and meditate holding the candle. this time, as you visualize/feel the energy enter your body, visualize/feel it traveling down your arms and pooling in the candle. when ready, place it on your altar and light the candle. as it burns out, visualize your desired outcome. let it burn out, or snuff it out, don't blow out the candle. finally, you need to put some effort in depending on the spell. if it was a cleansing, maybe put up some herbs to deter negative energy, but if it was a love spell, talk with people, socialize, instead of sitting alone with your head down. the effort you put in will increase the spells ability.