Hi Holly
It depends on the farm as to exactly what you'll be doing, but mostly it'll be assisting with morning/afternoon milkings and then any odd jobs that need doing around the farm. Some farmers will let you just come for morning and afternoon milkings and go home in the day- depends on how much other stuff there is to do I guess.
Don't worry about looking stupid/incompetent in front of the farmer, the only time farmers seem to get pissed off is when you don't use your common sense (or when a farmer's view about what constitutes common sense differs from what you or I would consider common sense!) No farmer will get pissed off with you because you're inexperienced, they wouldn't take work experience if they weren't willing to teach you a thing or two!
Above all enjoy it, and its probably a good idea to write down a few things when you get home each day about what you've seen/done/learned etc. You'll need to call on this info when you're interviewed by the vet schools, and it makes it much easier to prepare for interviews when you've got a few notes from your placements.
I say that from hindsight of course, typical disorganised me didn't do that.