If you are trying to become a mermaid, that simply will not happen. It doesn't matter how much you believe in magic, or how many times you try to cast such a spell. You cannot become a mythical creature.
In order for that to work, you would have to change your DNA, which magic cannot do.
Learn what magic really is, what it can and can't do. Learn and practice the basics (as Rikki listed). If you are willing to put in the time to learn the basics of magic and spell casting, then you'll have little to no problems in the future.