it's like falling into an extremely big hole. my recommendation; flit about for the next term and just sort your ideas out. there's no rush.
you can choose from quite a few mediums, but i can't be bothered telling you what they are. short story, critical response and poetry would be the most common (but this doesn't make them 'bad' choices). you can change between mediums and conepts until you hand the thing in, but it's always best to go towards something you will have a SUSTAINED interest in. it's not too much effort compared to a lot of other subjects but really requires that you don't slack off (even then, this isn't a huge dilemma).
in terms of 'support' this will really depend on your your teacher. i had a great teacher but our lessons were erratic, i was lazy, she was lazy. it was all nullified by the fact that our class was composed of two people. it was really scary handing over some of my stuff to her though. if you're not much of an exhibitionist, it helps that you trust your teacher not to laugh at you. i'm actually being pushed into doing writing after the hsc, so i ended up with a little too much support. but of course, you may get a teacher who will read through your drafts, give you a sticker and pat you on the head (although i've heard this is illegal?).
it's a really independent sort of project, but you'll be looking for opinoins and direction. if your teacher sucks, ignore them. get on with it and ask people to read through your work, they HAVE to be critical and sort of anal. there's no point handing it to your friends, because they will all tell you how talented and wonderful you are. while it's great to finally be recognised for the literary genius that you are, it is not constructive at all.
moral of the story; do the course. it won't kill you. if you're really unsure, have some back up units, but people rarely go completely crap in this subject anyway.