Well, I answered much of this question with this answer, which is a more general question about beginning writing.
So read that, it will get you through the first chapter. Now to be more specific to the question of writing fantasy, since you are having ideas about the story, I would write them down as notes, on index cards.
For a fantasy, you will need to figure out a "system" of magic: Who can do magic (is it hereditary, or random, or everybody?), where the energy comes from, and in particular the limits of magic. You can leave much of this up in the air, but typically you don't want an "anything goes" flavor to the magic or the story ends up just random things happening, with no structure or reason. So think of magic as just another form of unexplained technology, not all powerful, but useful in certain circumstances.
You can derive that from your ideas about the story if they include magical things happening. If your ideas take place in more than one venue, if your characters must travel, you will need to build the world to some degree, either as you go or up front. If you don't want to get into that too deep, steal the climates, mountains, forests, deserts, rivers and lakes from some real place on Earth, where the cities are IRL is a realistic place to put your own cities, big or small. Crib from real life and you don't have to turn yourself into a geologist, climatologist, botanist and wild-life expert.
Try to put your ideas into scenes you want to show. Then sort them out, into four bins. A story progresses through basically three acts (see this other answer of mine.)
I call them Act I, Act IIa, Act IIb, and Act III. Each of these is approximately the same length. So take your ideas, and sort them out to where they might belong in these 4 segments, and in which order they should occur within that segment. You can figure out how to incorporate them later, but just knowing they are there you can work toward them as you write.
Act I is the MC's normal, stable world (Any magic and any supernatural elements should be introduced here, or at least hinted it strongly). In the middle of Act I (1/8 of the way through the story) will be the beginnings of an "inciting incident", something that is going to take the MC out of her normal world. By the end of Act I, (1/4 of the way through the story) this inciting incident makes her leave her normal world (physically, emotionally, mentally, or any combination thereof; she is forced away from the status quo life).
Note that in a 100,000 word story, the first 1/8 is 12,500 words. In submission format, that will be 50 pages of text (250 words per page). My first link should help you get through that; note your big plot-point inciting incident hasn't happened yet, this first 50 pages is all about your readers getting to know your protagonist, how the MC thinks and acts, and start getting to like the MC as a person. This is very necessary or the readers just won't give a crap about the MC's problem when it does come along, in the next 50 pages. You need the readers invested in the MC, their love life or lack thereof, what is important to the MC. They need to care what happens next.
Act II is about solving the big problem introduced in Act I. Typically the first half of Act II (Act IIa) is a 'reactive' phase of problem solving, the MC tries things and they aren't working very well. She is fumbling about, failing, but learning about the problem. Often the reason she is fumbling about is because she misunderstood the problem, she thought it was one thing, and it is turning out to be another.
If you know what they problem is supposed to be for your plot, try to think of a way it can present itself, to the MC, as something else, try to think of a way that for Act IIa, she is failing because she is trying to solve the wrong problem. An alternative is just that she is trying quick fixes, or brute force methods, and these don't work because the problem demands a strategy.
Act IIa ends with a "turning point", either a failure or an epiphany or both. Act IIb is about a proactive phase, because NOW our MC knows wtf she is doing. She may not have the solution yet, but she knows what she needs to learn (and can still be failing), and she has a plan to learn it, to acquire resources (including other people with talents, perhaps, or some magical artifact), etc. She's on a mission.
Act IIb ends with the final turning point, things are in place and she is ready to kick some ass.
Act III is the story of that; resulting in the finale. Typically she will be triumphant, or at least mostly triumphant, and that leads into the denouement, where everything is wrapped up, explained, and the MC transitions to a new "stability", either her old normal world, or a new normal.
You don't have to know all of this to start writing, as a discovery writer I always have a strong sense of who my MC is (in my head, I don't write notes), and I have a strong idea of what the big problem will be. Then I start writing my MC solving day-to-day problems in her normal world, which everybody experiences.