Think 'appropriate for work'. Ask yourself, would someone working in an office wear this? Would a teacher wear this? Use that as your guide. But as for how formal, that's entirely down to personal taste and what you feel comfortable in. Some guys like a suit and tie, some prefer chinos and a chequered shirt. Some girls like pencil skirts and heels, some prefer... chinos and a chequered shirt. It really is up to you. Wear what you feel good in.
More important is what NOT to wear. Excessive jewelry is a no. And bear in mind that if you wear a tie or a watch you may be asked to take them off (fine in GP, but not in hospital due to bare below the elbows and infection control concerns).
General tips... The most important thing is to remember that patients are people, and be nice to them! A bit of friendly small talk can go a long way. Ask them how they are and about themselves etc.
Remember to introduce yourself and make it clear that you're a first year medical student. The patients will probably have been told that you are new to this, but if they haven't it is okay to tell them yourself. They usually get you to speak to really nice friendly old people who aren't scary at all, but they'll be extra nice if they know why you're a bit lost.
Good luck, you'll be fine