Good job for recognizing that nails need trimming. I see so many dogs with mangled feet and early arthritis due to not clipping the nails.
Are her nails black or clear? If they are black really all you can do is get a feel for it after clipping a while. Clear are easier as you can see the blood vessels. Trim very little off every few days until her nails are no longer touching the ground. Each time you trim the nails, the quick will recede a little.
http://www.vetmed.wsu.edu/ClientED/dog_nails.aspx
http://www.thepetcenter.com/gen/nailtrim.htm
Those pages tell you everything you need to know with pictures. If you are still worried about hurting her take her to a vet or groomer but ask them how they clip nails. Some people will just cut the whole nail off and let the dog bleed.
Be aware if you do cut a quick it won't hurt her too badly. It's akin to cutting your own nails too short. Yea it hurts but it goes away quickly. Almost every dog owner has quicked their dog at least once. I do sometimes as well with my Staffy Bull, she has black nails and throws a TANTRUM. So occasionally she gets quicked but she never notices she has been quicked. (Which is why she should be throwing the tantrum...but alas it is just drama)