It's good that you're starting with the story! Getting straight into the action is my favourite kind of storytelling (and I believe that's the general opinion nowadays?)
I've seen this approach taken often in your situation:
1. Slowly reveal her backstory through hints and implications in the rest of her and other people's actions and interactions
2. You can also (after a little bit of immersion in the main story) lay it out explicitly, by having her recall her initial motivations.
1 can be achieved by (for example) describing her habits, ticks, and mannerisms. Unique behaviours that have become ingrained in her being can be used as a clue for the reader to guess at her backstory. Of course you can mention the origins of the habits as well to make sure the audience ends up accurately on the same page as you, or let it remain mysterious and up to the reader's deduction. You can use interactions with other characters to similar effect: making other people who knew her from before run away from her when they see her waving to them, or talk to her referencing some childish nickname she used to have.
You can do 2 by simply having her talk about her history with someone she's come to trust. 38
That's quite common and (for me) touching, maybe others find it cliché and cheesy. You could also easily internally monologue it, and it often ends up as quite an epic reading experience to be shown more depth, or perhaps the new surprising truth, of the character through that character's deepest truest thoughts.
I personally find method 1 more exciting. Don't ever feel that you have to share the backstory with the audience. It's very important for you as the writer, but may be boring to explicate to them — and completely draw them out of and put them off the story you want to tell. As long as you keep their origin in mind yourself, the audience will be able to see the kind of person they are, and perhaps infer their backstory, just through how they and others behave in the main story, and this is far more gripping, and a far more fun experience (at least for me).
Of course, if you are dead set on explicitly explaining parts of their backstory to the audience (first ask yourself why?), then aim for a clever balance of 1 and 2. 2 is not itself bad (I do think however that prologues are bad!), it's fine to state things about backstory, but be careful not to get too sucked up into it, dedicating too many words for it, it can really slow down and damage immersion if overdone. 846