Was browsing the forums, and thought 'Hey, that username sounds familiar.' :P I'm not much of a GM, and I might be biased since I like your idea, but I think I can be of some help.
The first thing that stands out is that it seems a bit bare, which is probably where learning how to build a world would help. However, I also like it because it seems there's more freedom for players to come up with things of their own rather than rigidly living in the GM's vision. You have to strike a balance.
Your overall basis is that beings are turning into monsters. Why? You say that there's "[a] chemical released by a plant or in the air", but which is it? And more importantly, is the origin of this chemical known to Terra's inhabitants? I assume not, because the focus would (or at least should) be more on destroying the source than destroying the monsters, meaning more plant destroyers than monster hunters. If no one knows, just say something like "beings are turning into monsters for unknown reasons" or "it's speculated that a chemical released by a plant is responsible for the monster outbreak". Even if YOU know the real cause, you should only give as much information as Terra's inhabitants know, and specify if they know (or think) it as fact, if it's a rumor, etc. This also helps characters that join the RP from knowing information that they shouldn't.
You should also elaborate on how the monsters are affecting the world. Say it's not safe to travel without a weapon... then there's probably a lot of blacksmiths and craftsmen to provide them. Maybe it's even rare to hear about someone who doesn't have some sword or dagger on their person, even if they're just walking to their neighbor's house. How many monsters are there, and how dangerous are they? How dangerous do the people see them? And what makes one a "monster" anyways, when there's also a reptilian-men race about? Is it because of their looks, or their behavior? I have no idea what a monster looks like. Are they big and hairy? Small and scaly? Varied? Does a human-turned-monster look any different from an animal-turned-monster?
You also seem set on this old sanctuary. Describe that a bit more, maybe give it a bit of history. I assume that Anna's going to do something in this sanctuary in the opening post, so maybe add in her history why and how she's come to be there, and also how the monster outbreak affected her (I assume she's some monster hunter, but it's never stated). This lets players see that she's not some stock character, but someone custom-made for the RP.
There's a lot more that goes into world-building, but I think that's a good start. If you want, we can work on it together or something, especially because there may be some cases where you can ignore what I just said.
Beyond world-building is presentation. When you're describing your RP, you use "I" and "we"; I think it's better to stay in third person (no I, we, or you). It gives it a more formal feel, and allows you to take the things you're not sure about out of the overall description, like when you're asking if monsters should approach after the bird flies away. You can add things like that in another section.
To illustrate, say I was making an RP. Under the introduction I'd write something like...
When the terrible tyrant, George Henry III died, the denizens of Kardia broke into celebration. However, their happiness waned as they realized his son, the new king, was just as evil. [blah blah, explanation of why this is important and what's happening now, blah blah].
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Overall, I figure our characters would be a part of a rebellion of sorts against the new king. There'd be lots of fights and drama and stuff. So we'd start by making our little rebel group and go on from there. [insert other stuff I personally want to say here]
... Okay, not the best example, but I hope it got a little bit of my point across. Think of it like separating IC from OoC; your description should be "IC" while things you're personally concerned with is separated as "OoC".
And I think that's enough for now. Sorry for writing so much. =~=;;