What should i use for guinea pig bedding?

Use hay! Usually timothy hay is the best kind!

Along with timothy hay another good bedding source is Kiln dried Pine Shavings. BEside being a comfortable way to sleep, it is cheeper than most bedding and guinea pigs don't need to eat it so unlike timothy hay they never have to worry about eating their pee or poo. Because that would be very nasty. Don't ever use Cedar Bedding thought it is bad for guinea pigs and can KILL them :( The cedar bedding is bad because the strong smell can cause respiratory problems.

Guinea pigs can also be kept on wood shavings (not sawdust!) and many companies also sell large bales of shredded cardboard which makes Excellent piggy bedding (this is often sold for horses who are allergic to wood shavings and is usually available mail-order)