Questions about nervousness and breath control

I'm joining my school's choir next semester, I have I think an okay voice(they don't care too much at my school), and at the beginning of whichever semester you're starting at, they make you sing a song of your choice in front of the class. I don't know what I will sing yet, but my problem is that even speaking in front of more than a few people makes me lose my breath. I can barely get through a sentence without running out of breath, even if I'm not very nervous, and I stammer a bit as well.

So my main question is, how can I reduce anxiety and improve my breath control, as I'm sure it will be even worse when singing? Sorry, I hope this is clear.