Nothing comes from nothing. Your magic must come from something, must be caused by something. You must have a reason why your world has magic. You don't have to state your reason, but you have to know it.
Every action has an equal and opposite reaction.
When your characters start using magic, they must be made to pay for it in some way. If it is effortless, it isn't real, and no one will believe your story. If your hero does good magic, the use of this magic will have a detrimental effect somewhere. Conversely, your villain's magic will have good fallout somewhere. Actions have consequences.