Usually, they don't care much about skill in guard tryouts, so don't worry so much. They make horrible people amazing, but it's not just the instructor, it's also the person's dedication, so keep that in mind.
You'll make it if it's not a big guard, and even then, you still have a high chance of making it.
Guard tryouts in general, well...if you mess up or anything like that, or you're just plainly horrible, they can't say anything during the actual audition, so don't get nervous about that. Don't go in there and be timid and nervous, 'cause they obviously won't think highly of that.
Really, guard tryouts aren't that bad, and besides, new people are usually horrid, but that's because we all start somewhere. They'll most likely take you in anyway.
Oh, and if THAT somehow doesn't work out, you should go for pit.
Oh, and don't follow Expert Village.
Oh, and Bassoons are amazing, aren't they?