Casting spells is an art, somewhat similar to learning a musical instrument. So, if you were to learn, say, guitar, you would start off with some finger exercises and simple counting. Then you would get t grips with the notes.
Spells work in a similar way. Instead of finger exercises, you exercise your mind to focus, - the exercise is meditation. Also, instead of learning the notes, you build up your ability by exercising visualisation. And so on, until you are prepared for what you are doing. Research "the basics" on this site.
Once you are prepared, I would personally suggest a basic protection spell to start with. :)