Make sure they know;

Where they are going and how to get there (bus times, fares etc).

Who they should ask for when they arrive.

Time they start and finish work

Safety - On their first day, they must find out/be informed about fire/evacuation/safety procedures

Arrangements for breaks and lunches (do they need to take their own sanwiches etc. or money to buy from a canteen/shop?

What they should wear (will they be getting dirty or be on their feet a lot?

See that they;

Get up in time to arrive on time.

Take with them everything they need.

Get enough sleep - they will be surprised at how tiring a day's work is and they do not want to appear tired or lethargic.

Finally, encourage them to talk about their experiences at work, what they enjoy and what they don't.