So you want to get published? Have written that amazing story and are now looking for a way to get it out there. Conferences are great, but not everyone can afford them.You're confused about when you should look for an agent... in fact, you're confused about a few different things. Mainly, that is how to impress agents and editors.
My research on the subject has lead me to this conclusion: They like to see that you have won writing contest and had other writings published. This can range from short stories to school papers, any writing that has made money (whether for you or someone else) is impressive.
What does this mean? It means that you write for your school paper, your local paper, an online magazine, anything that showcases your ability to write. You try contests (remember, don't pay an arm and a leg for something you are not sure of) and see what happens. Below are some things that I vouch for.
Novel Rocket Launch Pad Contest: I did this last year and am planning on it for this year as well. Though I did not win, I got valuable feedback on what I sent in that has helped me out. This year it does have a $45 fee though, so only send in something that you are sure of advancing at least to the second round. It doesn't say so on this page, but past winners have been published with such companies as Thomas Nelson and others.
Five Glass Slippers: This is a contest and a way to get something else out there. Five short retellings of Cinderella will be chosen to be published in a book. The fee is only $10 and it is edited by Anne Elisabeth Stengl, one of my favorite authors.
Jill Williamson has this page on her blog of magazines and such that need writers if various ages. Jill is another favorite author of mine and is dedicated to helping teens get published. Though I have not tried these sights, I trust her judgement.
There are many other contests and magazines out there, but these are what I was able to think of. Hope they help!