Well, by looking at your profile I'd say you are totally new to this since you do not state how long you've been practicing.
Therefore, I will tell you what everyone else tells all the newbies. Study the basics before you cast a spell.
Search the forums for "basics" to help get your started. You should look into the basics of meditation, grounding, centering, visualization, and focus. Your mind must totally clear if you ever want to cast a spell.
This is not done over night. Do not rush it. It takes many years to be powerful in this craft.