I'm presuming guinea will have dried food with him/her.
So you just need - hay, loads as shehowines said. They eat practically all the time.I clean our boars outdoor house every 3 days completely, pick up wet bedding daily.Inside mine have cat-litter and newspaper with a little fleece in a box each, and nice hay. You'll probably get away without cleaning the cage over the weekend, depends how messy your GP is.
Water bottle /bowl, make sure it's topped up with room temp water (I leave mine in the kitchen overnight rather than straight out of the tap)
There's a GP veg list on here to give you ideas but just avoid iceberg lettuce, potato, onion, not too much apple, grapes-only one or two.
The little monsters can escape, so if you let him 'free-range' make sure he can't make a break for it or chew anything.And they will chew- books,papers.clothes,electric cables, they don't discriminate!
Lots of cuddles, and if they do a little dancey-dancey- stamp routine, they want a pee. They'll just pooh with abandon.
Pick them up so they feel secure, hand under the stomach and under their rump., then hold on your shoulder or on the crook of your arm.
One of my guineas jumped out of my arm to get into his outdoor cage, so hold tight when you put them back.You wouldn't think they are capable of acrobatics but they can surprise you!
Enjoy him, you'll love him. You might even want one yourself 
Cor that was an essay.