guinea pigs need to have their nails trimmed every month or so. You have to be careful not to cut the quick. Most vet offices charge around ten dollars for a nail trim.
If you take your pig to the vet for it's nails they could probly show you how to cut them properly.
Long hair guinea pigs will need a comb to get out knots. For short hair pigs you can use a soft bristle rabbit or ferret brush.(I have a Super Pet "ferret petting brush")