This is a picture of half of my cage and my Guinea Pig sleeping.
You need a bed, most cages come with a Plastic bed, and its best to fill this area with hay for the Guinea pig to sleep on comfortably.
To fill a cage.
Use some newspaper that is at least a week old, or two is best, as the ink isn't dangerous if eaten as its old. Fill the bottom cage about 5 pieces of newspaper thick.
Cover this all with about One inch of wood shavings (wood shavings only, can be scented wood shavings such as lemon, lavender, or apple). No cedar or pine shavings and no sawdust. Care fresh is good too, as it prevents allergies, and is odour preventing and soaks of 3 times as much as conventional bedding.
Then in the bed, fill it about 6inches deep of hay, so they can bury themselves in the hay and keep warm. Also this will be used as food and needs to be topped up daily.
Use a ceramic bowl for food and a water bottle attached to the side of the cage. Try to use some tunnels or toys in the cage such as balls, and chew toys to make it more interesting. Guinea Pigs are inquisitive animals and new things in the cage are great to get their natural instincts going!