Your main character absolutely MUST be a teenage girl. Ideally she should have a traditionally masculine identity, such as a warrior or hunter, and she must be proficient with weapons. A bow would be best, but knifes are okay too. She should have a tragic backstory that probably includes the death of one or both of her parents.
If you can come up with a unique magic system that would be best, but the most important thing is that there is some impediment to magic use. Possibly magic users are persecuted, maybe magic is something only a very few understand, but more likely magic has mysteriously or sinisterly disappeared. This should be an especial problem for the main character and her family who have depended on magic in the past.
This should be high fantasy, but it should probably be based on a culture that isn’t white so that you don’t get compared to Sarah J. Maas or J.R.R. Tolkien. Urban fantasy isn’t particularly popular right now, probably because the market was so over-saturated with it in the early 2010s. So find a way to make high fantasy unique.
The villain needs to be a king. A queen would be okay too, but either way they have to be SUPER evil. Completely one-dimensional, over-the-top evil. Ideally the king/queen is the reason magic has disappeared, but it’s also okay if they are suppressing the populous. They should be fabulously wealthy, especially compared to the main character who struggles to get enough to eat. And they should have a giant palace in the capitol that is far enough away from where the MC lives that it will be a journey to them, but close enough that they can send soldiers easily. And one more time, because I can’t stress this enough, the king/queen apparently has zero character motivation other than that they are evil. Maybe they’ve been corrupted by dark magic, or maybe a tragic past has turned them into a monster, it’s up to you really.
The Best Friend
Your main character is mostly a loner, but they should have one or two extremely close friends. This might be an older brother who is protective of them, or a younger sister they are super protective of. It might be a young man who is obviously in love with them but they have only a platonic interest in. If you can make this into a love triangle that would be great, but nobody likes it when the MC ends up with this character in the end, so they can’t be the main love interest.
This is the most important part of the book other than the main plot, so it’s very important to get it just right. The romance should have sparking chemistry, but for some reason the characters can’t or won’t admit their feelings for each other. Ideally this is because the romantic interest is the son of the evil king/queen. Maybe they hate magic users because of some past trauma and have to learn that the MC isn’t evil just because they’re a magic user. Maybe they think they’re incapable of love. Make sure to have some obstacle in the way, we don’t want any insta-love here!
The plot of the first book in your series (because this *is* a series) really depends on how long this series is going to be. But some ideas are: bring back magic, take down the king, some kind of competition, rescue the sibling, and find the McGuffin. If this is a duology you should probably plan both books now. If it’s a trilogy or longer there’s not really a need to do that as long as you have the general idea of how the whole thing is going to start and end. Just make sure you pick a plot that can resolve in the first book (unless its a duology!) but will leave the broader plot open for more books.