Yes, you will probably need to trim your guinea pig's toenails,
     unless he or she does a lot of running around on bricks or
     concrete or other rough surfaces that will keep the nails short.
     Once the nails start getting long there is nothing but you
     clipping them to remedy the situation; the nails will eventually
     either curl back into the pad of the foot, crippling the guinea
     pig, or else break off and sometimes cause bleeding and
     infections in the process.  You can clip the nails at home
     yourself or, if you feel insecure about it, you can have a vet do
     it the first time so you can see how it's done---although they
     may charge you a fair bit for this.  You can use either a normal
     human nail clipper or the clippers with curved blades they sell
     in pet stores for trimming cat nails.

     The easiest way to do this is to have a friend help you, so that
     one of you can hold the guinea pig while the other trims the
     nails.  If this isn't possible, some people recommend rolling
     your guinea pig up in a blanket or something, so he (or she)
     can't see and doesn't struggle, and turning him on his back in
     your lap so his face is still covered but his feet stick out.
     I've never tried this myself, so I don't know the precise
     logistics of it, but apparently it keeps them from putting up a
     fuss.

     The thing you have to be careful of is not to cut the quick,
     which is the pink part in guinea pigs with white nails.  Just
     like in humans, the pink part shows how far the flesh of the toe
     extends, and the white part has no nerves.  If your guinea pig
     has dark nails, you may need to use a brighter light source to
     see the quick, which should be slightly darker than the end of
     the nail.  If you still can't see where the quick is, just cut
     the nails often and a little bit at a time and you should be
     fine.  If you do accidentally cut the quick a little and it
     starts bleeding, dab a bit of hydrogen peroxide on the spot to
     help prevent infections.  Try to hold him or her until the
     bleeding stops so that the site stays clean and the cut is given
     a chance to heal over somewhat.  There are products---"Quick
     Stop" is one of them---that you can apply to the site to help
     stop the bleeding; these are helpful (but not necessary) in a
     situation like this.