I know you should have a strong need/conflict for your main character that carries to the end of the story, my question is how early does this have to be introduced? In my story My heroine has a driving need to find her identity and to become a warrior despite great opposition. This is pretty clear from the very beginning. The main romance/conflict begins a bit later in the story, when the girl-meets-boy-meets-big-problem-they-have-to-overcome. There are lots more things going on than that but i was trying to work out the main underlying conflicts.
It sounds like a conflict that she wants to become a warrior, but the society opposes it (omg, women shouldn't have muscles it's ugly! or something like that). Her goal is to overcome it. The reader looks forward to finding out how she does or doesn't reach it. To me that'd be enough, but maybe some other readers would like her to also look for some magic diamond that will save the kingdom. First of all you should write what inspires you and not stuff the magic diamond in there if it doesn't fit. You can start the story with her e.g. trying to join the army or get herself a fencing master, but it's denied. You can start it with the village boys picking on her "manly" biceps and she beats them up and then swears that someday she'll really show them all, and then she embarks on a journey to become a warrior. But it sounds like you already have it down, you introduce a conflict at the very beginning. Some stories start slower, which is also fine although it might put off some readers, as long as the writer has a good reason to start the story the way they do.
When you enter into the main meat of your plot is up to you, as long as what you write before that is not one long dip. You can for example have the beginning of your book about your heroine's early years and her fight to become a warrior, and then write about your main conflict/plot after that. Character progression is often as important as your main plot, so as long as what is happening is going to interest the reader you can't go too far wrong, as long as you don't leave it so late to switch to your main theme that you end up with a rushed and short plotline.
Each conflict defines a plot. But you are not limited to a single plot to define a story. You can have a chain of plots. The connecting fiber can be the relationship between characters, or it can be a natural progression from goal to goal. The tension should continue to escalate up to the climax, but instead of a continuous rising slope, it could be like waves crashing against the shore as a storm approaches.
Consider more complexity in the goal, the MC's external goal is what grows the story, but the MCs internal goal (one she may not at first recognize, and the reader may not either) is where the resolution actually lies. For example, (and your story may differ considerably), finding her identity and becoming a warrior despite great opposition is the external goal. The internal goal might be coming to terms with her fear being a warrior makes her masculine. Overcoming it might be discovering female and powerful are not mutually exclusive. Makes it harder to write, but a better story.
I like that , Ginger, actually I already have a little bit like that written in, but I hadn't really thought about it much. Later when she has done most of her training she has gained the acceptance of the 'men' but realizes they don't view her as a woman exactly anymore, she gets really angry. There is a funny bit where she has to dress as a boy to keep herself safe on a journey and she realizes how much she has to fight for who she is, against the perceptions etc. Cogito, I thought my story would work like that and had a stroppy person tell me a chain of plots wont work, but you make it sound doable. In a way my book is two parts, the childhood/training/becoming a warrior part, than the adult fight for her kingdom and her [forbidden] love. If the character is developing and it's exciting to me, it should be interesting to others, right?
I think KaTrian, Cogito and GingerCoffee answered the question well. The only thing I would add is that I think people make the mistake of defining fantasy as 'sword & sorcery' epic adventures. Fantasy does not have to have these elements and I would argue that the best ones don't rely heavily on the 'fantastical' gimmick elements of the story but rather the relationship between the characters and the way the journey changes them. The driving force of the novel should the thing that the readers care most about and for me as a reader, that's usually the characters.
Multiple plots are some of my favourites. A way I like to approach it is to have one underlying plot with several others more prominent. For example one of mine that's in progress has the MC seeking world domination which is the main plot. Then there's the conquests, keeping the populace happy and then his switching of goals after his demon father decides to attack him.
