Typically guinea pigs do not need baths nor do they like playing in water. Bathing can be done periodically to ensure any 'messes' in their hair can be cleaned up. Nails can be trimmed as well if there isn't sufficient wear on them in their daily activities. Cat nail clippers work fine to do this, just make sure you don't cut too far up the nail into the living part of the nail or quick.
I am not sure how much research you have done so far on guinea pig care so I included few good links that are most helpful, including one about building your own cage.
www.guinealynx.info/healthycavy.html
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PHLdyPayne
