The size
First off, the size of the cage depends on how many guineas you have in there. Please try to make it as roomy as possible because the guineas start to feel cramped and unhappy if the cage is small; No one likes an unhappy pet. So if you do notice your friend is having some trouble getting around, it might be time to get a bigger cage because guinea pigs grow fast. I do understand that the cage takes up space so in that case, let the guinea pig out of the cage for a little; they enjoy discovering.
The bedding
The best getting that you can used is not found in pet shops but is very easy to find. If you get a regular fleece blanket that's thin, it will work perfectly. All you have to do is every time you wash it, put white vinegar on the blanket and wash regularly with soap, but do not use fabric softener (the vinegar will help the guinea pigs urine to go through the blanket instead of laying on top). Since the urine will be going through the blanket, you would need wee-wee pads (dog pads) to put under the blanket, hand towels will work well too. Please do the guinea pig a favor and do not use wood chip bedding. It tends to start hurting they're feet when walking and their body when laying down. This also attracts flies and flies tend to lay their babies, maggots, into the cage. That is very unhealthy for your guinea so I strongly suggest you dont use it. There is also another bedding made from recycled cardboard. At first i thought it was a good bedding but one of my guinea pigs, Leo, died because the bedding was carrying to much dust so please don't use this either. It will be best to clean your cage twice a week, and if you can, spot clean (picking up feces but not to full clean the cage. I use a sand shovel when doing this) everyday.
The Accessories
Like every pet, you need a food bowl and a water bottle. Guinea pigs like shelter so get a den (mini house for rodents) for each guinea pig. An extra dish might be in handy for hay or guinea pig snacks (see the food for your furry friend tab).