Magic is often central to many fantasy worlds, but it can be difficult to come up with something fresh and new. It's very easy to get stuck in what's been done in the past or to lean on the magical theories of our world. Often fantasy cultures think about these things the way we do, whether it’s based on the four elements or the ritual magic of drawn circles and lighted candles. However, these are all concepts that have been explored before, and unless you can put a truly original spin on it or add depth, it probably isn't going to do anything for your story.
It might not hurt anything, but it won't add much either. However, an interesting or original magic system can draw the reader in, characterize a world, and add depth to a culture. The best way to do that is to make it as deeply a part of the culture as you can. The more entrenched in your world the magic is, the more unique it becomes. When you tailor even a general magic system, it shows it in a way that is new and different. Especially if you've given depth to the cultures and/or species with which you've populated your world.
The basic magic questions are fairly straightforward. Where does it come from? What does it require? Is it a learned skill that can be practiced by anyone, or is it something only some people can do? Is it a limited resource, or an infinite power?
But there are other questions that deserve answers which are just as in-depth and thought out. Consider how your characters feel about magic. Not just the ones who use it, but those you can't or don't. What does it require of those who do use it? How does it affect their daily lives? What about the daily lives of those who can't or don't use it? How do their daily lives affect it? What terms do they use to define it? How does it connect to the other aspects of their lives? If asked to explain it to a child, what would they say? As Einstein said, if you can't explain it to a child, you don't really understand it yourself.
Let's start with terms. There are a few things that you'll likely need to find terms for when creating a magic system. What are the people who use magic called? Is there a name for those who can't or don't? What is the act of working magic called? Sorcery? Enchantment? Do your people cast a spell, or summon the elements, call forth the powers? What is an individual work of magic called? A spell? A charm? A working? Are there any objects needed? Does it require strict rituals or certain materials? Words? Gestures?
The most important thing, in my opinion, is to entrench the magic in the culture or species you're creating. If they're fond of singing, perhaps they were their magic in a similar way. If they're big on math, perhaps those are the terms they use to describe what they do with magic. Do they think of it like science or like religion or like art? Is it a means to an end, or an end in itself?
Different cultures from our world have different ideas about magic, and while it is often linked to religious beliefs and rituals, it doesn't have to be. Alchemy is an interesting blend of science (chemistry) and mysticism, and even scientific minds such as Isaac Newton found it a worthwhile pursuit. While many magic systems require rituals there's a distinction between ritual magic and religious ritual or psychodrama. Not every culture will link magic and religion and there may even be conflict between the two--as there has often been among cultures on Earth.
Magic isn't always nice, either. And by that, I'm not referring to "dark arts" or "necromancy" or uses for magic which may not be culturally approved. Often we see the ability to do magic as lifting one section of the population above the rest, unless it is outlawed. However, there is always the possibility that instead of being respected for their ability to do magic, magic-using characters might be reviled for it. That could be because it's considered a defect or because the materials necessary are considered dirty, or simply because magic itself is considered base. All of these factors can help to add a deeper element to even a simple magic system, making them feel fresher.
So, when creating a magic system, think a bit outside the box. Perhaps your culture doesn't recognize four elements, but eight. Perhaps they consider it a discipline that only scientists engage in, or they limit its use to only particular activities. Perhaps no one can have a spell worked on them unless they have consented to it, or perhaps enchantments laid upon a person can be traded and have become a separate form of currency. There aren't any limits when it comes to magic, which makes it a great place to really stretch your creativity!