Almost no companies accept unfinished manuscripts, and besides, you may wish to change things, so finish writing first. As for the actual writing, different methods work for different people. Some people like outlining, some people like freewriting. Personally, I hate outlining. Experiment to figure out what your style is.
As for length, you should write until it’s done, but try to push it past a couple hundred pages, because those have a better chance of being published.
A note about agents. With them as well, you need to be finished writing before you send them your manuscript. They can help a lot with the process, but are expensive, like I said, and if you can’t afford it and you’re willing to work a little harder, you can do it yourself.
Don’t self-publish. You have to pay for it yourself, and not only is that expensive, but it may not reach as many readers.
How often should you write? That depends. As often as you want and have time for, as long as it doesn’t interfere with the other parts of your life. No writing when you should be doing homework. :) Honestly, I don’t think it matters. Some people say you should write everyday for a set period of time, but that doesn’t work for me, so I don’t.
By the way, it may be harder to get published if you’re in your teens. But it may not, if the publishing company you’re sending to doesn’t care, if they just care you’re a good writer. Also you may be able to take advantage of certain publishers who publish teen. There’s one in Canada called Kids Can Press, and they may publish teen work too. You may be able to find something like that near you. Eithor way, be sure and tell your parents that you’re trying to get published. I’m not sure about this, but I think because you’re underage you may need their permission to sign a contract. And they may wish to hire a lawyer to make sure you’re getting a fair dear.
When you’re finished, you should get someone else to read it, just to get an unbiased view of it.
One last thing, just to clear it up. You don’t hire an editor. Editors work at the publishing companies. But when you send a manuscript out, you should know who you’re sending it to and be sure to spell their name correctly. Some editors won’t even look at manuscripts where they’re names are mispelled. And since each company is different, you should check out their website to see their guidelines. Some want one chapter, some want three, some want the word count, some don’t do. Doing a little research will be highly beneficial to you.
Hope this helps and good luck! Happy writing.