How can I look after my baby sunflower?

My office is having a sunflower-growing contest, and I am now the proud owner of a four-inch sapling. The trouble is I have no idea how to look after it. It will need to be grown indoors until it’s too big to be eaten by squirrels. I have a nice sunny south-facing window. I know it will need a bigger pot but how big will it need? How much water? Do I need plant food of some sort to add to the compost? I seem to recall reading that used coffee grinds are good for roses, do they help other plants too?

I’m not expecting to win the contest at all, I just don’t want to kill it through ignorance