How did Skulduggery Pleasant start to mostrar interest in magic?

That's a good question, people don't have the answer for this yet because very little about him is revealed in the books, the mention was only in the first book when he told Val he got into magic, it was also revealed later on that he had a chrest and a decent family, probably a family of sorcerers, he may have grown up into the culture of magic and have interest on it since the begining...
any other scenario is possible we don't know yet.