When it comes to choosing whether to end an adjective with -ic or -ical, there aren’t really any overarching rules to help you. With some words, both suffixes are correct and can be used interchangeably, though one might be more common than the other—like metaphorical over metaphoric. One could even be so obsolete that it seems wrong: Scientifical, for example, is a real word, but people almost invariably opt for scientific these days.
In other cases, -ic and -ical are both correct, but the words they create have separate meanings. Historic vs. historical is a special situation, because it technically falls into both of the aforementioned categories. According to the Oxford English Dictionary, the two terms both mean “relating to history; concerned with past events.” They also both mean “having or likely to have great historical importance or fame.” In short, anything you describe as “historic” can be described as “historical,” too.
However, as Grammarly explains, historical and historic aren’t necessarily considered synonyms in modern usage. Historical is most often used in the former sense; historic in the latter. If, for example, you mention a “historic moment,” people will likely understand that you’re talking about a moment that made history because it was so important or noteworthy. If you mention a “historical moment,” on the other hand, people might just assume you’re referring to a moment that happened a long time ago.
As for whether you should say “an historic moment” vs. “a historic moment,” it’s up to you. But using an before h-words has all but died out, so you do run the risk of eliciting some eye rolls if you try to bring it back during a casual conversation.