I'm having some real dilemmas creating rules for magic in my fantasy novel. Here is my issue: there are three intelligent beings included in my novel. The entire premise is that all races were created by all-powerful beings, and with good intentions. Humans are characterized as being capable of anything they set their minds to. Because of the desires and free will of men, they are easily corrupted. That being said, the kingdoms of men fear magic due to its threat against them.
I'm using the cliché party of two men and one woman as protagonists. Each protagonist is from a different race. The woman is from the mystical race, and therefore will have been taught to use magic by her people. She is acting as the group "healer". Another character is from the human race, and will be acting as the team "tank". The third character is someone I am modeling from a D&D campaign I did where I made a changeling sorcerer. The "changeling" equivalents I am using in my world are frowned upon by the human race, due to them being deemed as untrustworthy because of their ability to change their appearance. Although they are not welcome in human society, it is not uncommon for these "changelings" to live amongst men.
The idea is that this character does not know his background (although he is aware that he is not human). He is adopted, becomes a sailor, and I want him to have the ability to manipulate weather so I can design him as his crew's weather mage.
So finally, my dilemma. This character would not have had formal training, which means he would have discovered this power on his own. The idea is that anyone in this universe has the potential to use magic (although some may not be very good) but I want the use of magic to be rare an extraordinary by non-mystics. Also, I should add that the antagonist is a human blood-magic user, which he discovered through experimentation and is the ruler of his nation, so his magic is also explainable. I've taken into account magic costs, through an explanation of a sort of naturalistic pantheism (all magic comes from the natural world itself). I guess my ultimate question is how do I stop every Average Joe from throwing fireballs when I am dedicated to the idea that everyone has the potential to throw fireballs?