Writing a novel or short story is totally do-able. However, sitting down before a blank piece of paper or word document can be overwhelming and discouraging. How does one make an idea become a 80-120k (or more) word book? Your English teacher had it mostly correct: outlines.
There are two ways you can do this:
One way is to buy a super fancy writing software that breaks down bits for you.
The other way is my way. You will need blank index cards, post-it notes, or envelopes. I used envelopes in this instructable. I like envelopes because you can use them to hold the parts of your novel associated with the envelope. I like to break it down into tangible, movable pieces. It makes the task less daunting and your progress more visible, increasing the odds you will finish.
Step 1: Characters and Word Count
Naming your characters and giving them some "life" is the first step for me. I find that the more I flesh out my characters at the beginning, the easier it is to write them in a consistent manner through out the novel. So do the basics first in that old "AOL" chatroom style we know and love :A/S/L. Age, sex, location. Physical descriptions. Any flaws they may have, make a note. That means if your character has a scar on the left cheek, you can always refer to his card/envelope and remember "left." For the photo I combined my protagonists, but for your own work keep one card for each, because it will become full of notes and changes you make later.
Ask yourself: what is their back story? How do they react in tough situations? That sort of thing. It is ok to think in general terms--characters tend to morph and change as you write them, developing their own voice. Just pull out your card for them, make a note and keep going!
Do it for your villains, too, if you have them. Any character you think will have repeated scenes. I like to do these first because I find as I create characters my story begins to formulate more in my mind.
Then do a card with a couple of places that may come up frequently and describe them. Again, continuity. In the photo I combined antagonist and scene--don't do that. Yes, it is a large number of cards to keep up with, but you can quickly file through them and find what you want.
Also, set a goal word count. I find 80-100k words is good to shoot for. The key is to keep your writing from being aimless--you want to write with a purpose, and goals make this feel attainable and exciting. 100k is nice, too, because it makes percents easier. So if you have 20k words written, you are 20% done. It gives small feelings of accomplishment as your fingers hammer out your tale.