trimming the nails is difficult at first, but it gets easier. if the nails are translucent, you will see a pink looking blood vessel in the nail. this is called the quick. if you clip too far down and hit the quick, it will hurt the pig and make him bleed a little bit. if this happens, use corn starch or an anticoagulant powder available at pet stores and cover the nail in it. if the nail is black, shine a light underneath it so that you can see the quick.
the blood vessel grows as the nail grows; thats why it's important to keep it trimmed, because if it grow too long, the blood vessel will be harder to avoid.
use small animal or kitten nail clippers that you can also get at pet stores.
i like to have someone hold them and distract them with a carrot or something so they're still enough to trim. go very slowly, and cut little bits at a time so it's not too short by accident. i would look on youtube for a video.