Cage! Your guinea pig's cage should be big enough for some toys and food, but also have room for him to run around. You can see mine hiding. :) Clean the bedding AT LEAST once a week. More is better.
Food and water! (and one of his chew toys) (excuse the mess! I hadn't cleaned out his cage yet!)
They should always have a place to hide. A good house would be a wooden one that they can chew on too! You can see T-Rex doesn't wanna hide tonight.
Hay rack is a great thing to have. Guinea pugs need hay available 24/7 and a hay rack keeps it off the floor. :)
The best type of food for a guinea pig would be just pellets. They get the rest of their nutrition through fruits and veggies and their hay.
Veggies every day are a must! Fruits can be given as a treat. :)
If a guinea pig seems lethargic, it could mean that they need more vitamin C. If you give them a vitamin C supplement, do not give it to them through their water. You can't tell how much they take.
If your guinea pig seems to be acting differently or seems sick, take them to the Vets. Before the vets is necessary, call around and see who will see guinea pigs. Some places don't care for them.
Lastly, give your guinea pig cuddle time/ play time. :) that makes them the happiest guinea pigs. Some like cuddling more than playing. Mine does. :)
A great site to go on is GuineaLynx. It has a BUNCH of info about different illnesses and how to treat them, medical info, etc. basically, everything you need to know will be on that site.
GuineaLynx also has forums so if you have a question about your guinea pig, others can help and give advice. :)