Trimming nails is very important to the well being of a rabbit. Although some can wear them down naturally, most do not.
Rabbits with white nails are easy to do. Trim to the quick (pink blood vessel). Don't go too far or the nail can bleed.
Black nailed rabbits are a bit harder to do. You can take a little off each time so you don't hit the quick. You can also hold a flashlight up to the nail to see the quick.
Trim about once a month for most rabbits. Some need to more often, other don't need it as often. At the very least, check a couple times a month to see how long they are.
If you are uncomfortable doing it on your own, you can get your vet to do it (and show you how to do it). Some groomers can do it, but most are not experienced with rabbits. Breeders should also be able to show you how to do it, but they will probably not be willing to do it for you on a regular basis.