While it can be a good idea to test your manuscript out on your target audience, I wouldn't rely only on them. I'd try to find some beta readers who are older and a bit more experienced in writing. Beta readers aren't people who are simply willing to read the manuscript and say "Yes I liked it" or "No I didn't like it." Beta readers are people who are willing to read it carefully, and then talk with you in depth about any problems they may have encountered, any weaknesses at certain points, and also to encourage you about your strong points and offer suggestions on ways you can improve. This is an in depth process, and most young teens probably aren't good candidates for that.
In my experience, AW is indeed a good place for you to find people to critique your work. You can post parts of it in the Share Your Work forum once you've been around here and contributing for a little while. If you want an actual Beta reader for the whole work, I suggest you post a thread requesting such in the Beta Readers request forum. There, people who also enjoy fantasy and have experience reading it or writing it can offer to read and critique it for you.
Don't expect it to be a fast process. It often takes time, but trust me, it's time well spent. You will learn a lot, and your work will become better as well.
I would advise against posting a manuscript in its entirety on a website somewhere. It's not very secure, and while it's unlikely someone would plagiarize it in its entirety, I've simply found that posting stuff on a website rarely does much to help you. You want a dedicated reader to look at it, not some semi-interested person who's just surfing a web site.
Only send your manuscript to people you know if you've talked to them about it ahead of time, and they indicate they're willing to read it. Otherwise, they might get annoyed. Again, unless they're a writer or a very experienced reader and able to offer constructive, objective feedback, the use of doing this as far as improving your work will be limited.
I hope this helps you out with some of your questions.