If you want to learn lots about caring for your guinea pig, go read the guide. Come back with questions if there is something you don't understand.
Guinea pigs cannot be litter trained and the kind of litter used in litter boxes is dangerous for them (they may try to eat it). So you may want to remove that immediately if that is what you are trying. You CAN used the litter box as a hay bin however. A layer of wood stove pellets under the hay will help to absorb urine. Oftentimes the pig will poo and pee more in the hay area than the rest of the cage just because the pig spends more time there (they need hay available at all times!).
The very first order of business is getting your pig into a cage with the appropriate sized living space. Check out guineapigcages.com toward that end.