Hi Suze! Nice to meet you!
Okay, first: Breathe.
Next: Go ahead and begin writing your story. When you get it to where you
enjoy the product of your work (staying within the specified word count), then take a look at a possible fit. Do you have the "Magazine Markets for Children's Writers" guide? If so, take a peek in there and see which one you feel is the closest
fit. Don't worry about it being perfect, or you'll drive yourself nuts.
You can also post a request out here for one of us to take a look at your story and suggest some magazines for you to look into. There are a few people out here who are more experienced at this than you and I, and would be more than willing to point you in the right direction. Also, your instructor will most likely make suggestions for you along with her return critique of your work.
Try not to get too uptight about it. That's what this course is all about ... to teach
us how to write publishable material, and find good fits for our efforts. And that's what this retreat is all about ... support, support, support, encouragement, encouragement, encouragement.
You've come to the right place, Suze!